Add Advanced Docking System library

This library is a fork of the following repository
https://github.com/githubuser0xFFFF/Qt-Advanced-Docking-System
Development started from the following commit
1de42a9766134eecd2611c2b4e209d3e0ede74d2
Incorporate all commits until
3ffbbfb6d01ff211d8349027221a19b1419296b5

 - Rename variables and files to follow the Qt style guide
 - General code cleanup (remove goto, fix typos, add overrides, remove
   explicit slots specifier, replace string-based with functor-based
   connections, replace dynamic_cast with qobject_cast)
 - Replace most preprocessor instructions Q_OS_LINUX/Q_OS_MACOS with
   Utils::HostOsInfo
 - Remove all QT_VERSION preprocessor instructions below 5.11
 - Change loading and storage of workspaces. Store workspaces in
   separate file instead of a list in the Settings.ini
 - Add workspace dialog, model and view for managing workspaces
 - Rename XML tags and use enum/bool instead of ascii art/numbers as
   attribute values. Use base64 instead of hex for storing geometry info
 - Remove internal style sheets
 - Add more build systems (qmake, qbs)
 - Adapt copyright header
 - Remove unix specific build rules
 - Replace ADS_PRINT with QLoggingCategory
 - Replace Java-style with STL-style iterators

Change-Id: Icf8c2fbaccec9680df83c6e2100e3446a090a437
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/advanceddockingsystem/floatingdragpreview.h

@@ -0,0 +1,134 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 Uwe Kindler
+** 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 Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 or (at your option) any later version.
+** The licenses are as published by the Free Software Foundation
+** and appearing in the file LICENSE.LGPLv21 included in the packaging
+** of this file. Please review the following information to ensure
+** the GNU Lesser General Public License version 2.1 requirements
+** will be met: https://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 or (at your option) any later version
+** approved by the KDE Free Qt Foundation. The licenses are as published by
+** the Free Software Foundation and appearing in the file LICENSE.GPL3
+** 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 "floatingdockcontainer.h"
+
+#include <QWidget>
+
+namespace ADS {
+
+class DockWidget;
+class DockAreaWidget;
+struct FloatingDragPreviewPrivate;
+
+/**
+ * A floating overlay is a temporary floating widget that is just used to
+ * indicate the floating widget movement.
+ * This widget is used as a placeholder for drag operations for non-opaque
+ * docking
+ */
+class FloatingDragPreview : public QWidget, public AbstractFloatingWidget
+{
+    Q_OBJECT
+private:
+    FloatingDragPreviewPrivate *d;
+    friend struct FloatingDragPreviewPrivate;
+
+    /**
+     * Cancel non opaque undocking if application becomes inactive
+     */
+    void onApplicationStateChanged(Qt::ApplicationState state);
+
+protected:
+    /**
+     * Updates the drop overlays
+     */
+    virtual void moveEvent(QMoveEvent *event) override;
+
+    /**
+     * Cares about painting the
+     */
+    virtual void paintEvent(QPaintEvent *event) override;
+
+    /**
+     * The content is a DockArea or a DockWidget
+     */
+    FloatingDragPreview(QWidget *content, QWidget *parent);
+
+public:
+    using Super = QWidget;
+
+    /**
+     * Creates an instance for undocking the DockWidget in Content parameter
+     */
+    FloatingDragPreview(DockWidget *content);
+
+    /**
+     * Creates an instance for undocking the DockArea given in Content
+     * parameters
+     */
+    FloatingDragPreview(DockAreaWidget *content);
+
+    /**
+     * Delete private data
+     */
+    ~FloatingDragPreview() override;
+
+    /**
+     * We filter the events of the assigned content widget to receive
+     * escape key presses for canceling the drag operation
+     */
+    virtual bool eventFilter(QObject *watched, QEvent *event) override;
+
+public: // implements AbstractFloatingWidget
+    virtual void startFloating(const QPoint &dragStartMousePos,
+                               const QSize &size,
+                               eDragState dragState,
+                               QWidget *mouseEventHandler) override;
+
+    /**
+     * Moves the widget to a new position relative to the position given when
+     * startFloating() was called
+     */
+    virtual void moveFloating() override;
+
+    /**
+     * Finishes dragging.
+     * Hides the dock overlays and executes the real undocking and docking
+     * of the assigned Content widget
+     */
+    virtual void finishDragging() override;
+
+signals:
+    /**
+     * This signal is emitted, if dragging has been canceled by escape key
+     * or by active application switching via task manager
+     */
+    void draggingCanceled();
+}; // class FloatingDragPreview
+
+} // namespace ADS