ProjectTree: Merge descendants of hidden Sources and Headers virtual folders

Unchecking the recently added "Show Source and Header Groups" option removes
the Sources and Headers virtual folders from the Projects tree. This patch
additionally merges descendants of these folders with identical priorities,
display names and file paths.

For example, if a project contains a folder including both source and header
files, both the Sources and Headers virtual folders will have child nodes
representing that folder. Previously, unchecking "Show Source and Header
Groups" kept both these identically named nodes in the Projects tree (one
containing sources, the other headers). With this patch, these nodes are
merged into a single node containing both sources and headers.

Change-Id: I4786eee4a528ea141a7fe117e14a050f68411890
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/utils/indexedcontainerproxyconstiterator.h

@@ -0,0 +1,188 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "utils_global.h"
+
+#include <QtGlobal>
+
+#include <type_traits>
+
+namespace Utils {
+
+/// A class useful for the implementation of the -> operator of proxy iterators.
+template <typename Reference>
+struct ArrowProxy {
+    Reference r;
+    Reference *operator->() {
+        return &r;
+    }
+};
+
+/// Random-access const iterator over elements of a container providing an overloaded operator[]
+/// (which may return a proxy object, like std::vector<bool>, rather than a reference).
+template <typename Container>
+class IndexedContainerProxyConstIterator
+{
+public:
+    typedef std::random_access_iterator_tag iterator_category;
+    typedef typename std::make_signed<typename Container::size_type>::type difference_type;
+    typedef typename Container::value_type value_type;
+    typedef value_type reference;
+    typedef ArrowProxy<reference> pointer;
+    typedef typename Container::size_type size_type;
+
+    IndexedContainerProxyConstIterator()
+      : m_container(nullptr), m_index(0)
+    {}
+
+    IndexedContainerProxyConstIterator(const Container &container, size_type index)
+      : m_container(&container), m_index(index)
+    {}
+
+    reference operator*() const
+    {
+        Q_ASSERT(m_container);
+        return (*m_container)[m_index];
+    }
+
+    pointer operator->() const
+    {
+        Q_ASSERT(m_container);
+        return pointer{(*m_container)[m_index]};
+    }
+
+    reference operator[](difference_type j) const
+    {
+        Q_ASSERT(m_container);
+        return (*m_container)[m_index + j];
+    }
+
+    bool operator==(const IndexedContainerProxyConstIterator &other) const
+    {
+        Q_ASSERT(m_container == other.m_container);
+        return m_index == other.m_index;
+    }
+
+    bool operator!=(const IndexedContainerProxyConstIterator &other) const
+    {
+        Q_ASSERT(m_container == other.m_container);
+        return m_index != other.m_index;
+    }
+
+    bool operator<(const IndexedContainerProxyConstIterator &other) const
+    {
+        Q_ASSERT(m_container == other.m_container);
+        return m_index < other.m_index;
+    }
+
+    bool operator<=(const IndexedContainerProxyConstIterator &other) const
+    {
+        Q_ASSERT(m_container == other.m_container);
+        return m_index <= other.m_index;
+    }
+
+    bool operator>(const IndexedContainerProxyConstIterator &other) const
+    {
+        Q_ASSERT(m_container == other.m_container);
+        return m_index > other.m_index;
+    }
+
+    bool operator>=(const IndexedContainerProxyConstIterator &other) const
+    {
+        Q_ASSERT(m_container == other.m_container);
+        return m_index >= other.m_index;
+    }
+
+    IndexedContainerProxyConstIterator &operator++()
+    {
+        ++m_index;
+        return *this;
+    }
+
+    IndexedContainerProxyConstIterator operator++(int)
+    {
+        IndexedContainerProxyConstIterator copy(*this);
+        ++m_index;
+        return copy;
+    }
+
+    IndexedContainerProxyConstIterator &operator--()
+    {
+        --m_index;
+        return *this;
+    }
+
+    IndexedContainerProxyConstIterator operator--(int)
+    {
+        IndexedContainerProxyConstIterator copy(*this);
+        --m_index;
+        return copy;
+    }
+
+    IndexedContainerProxyConstIterator &operator+=(difference_type j)
+    {
+        m_index += j;
+        return *this;
+    }
+
+    IndexedContainerProxyConstIterator &operator-=(difference_type j)
+    {
+        m_index -= j;
+        return *this;
+    }
+
+    IndexedContainerProxyConstIterator operator+(difference_type j) const
+    {
+        IndexedContainerProxyConstIterator result(*this);
+        result += j;
+        return result;
+    }
+
+    IndexedContainerProxyConstIterator operator-(difference_type j) const
+    {
+        IndexedContainerProxyConstIterator result(*this);
+        result -= j;
+        return result;
+    }
+
+    friend IndexedContainerProxyConstIterator operator+(
+            difference_type j, const IndexedContainerProxyConstIterator &other)
+    {
+        return other + j;
+    }
+
+    difference_type operator-(const IndexedContainerProxyConstIterator &other) const
+    {
+        return static_cast<difference_type>(m_index) - static_cast<difference_type>(other.m_index);
+    }
+
+private:
+    const Container *m_container;
+    size_type m_index;
+};
+
+} // namespace Utils