Factor an overlay widget out of progress indicator

The principle of having a widget on top of all other children of the
same parent, for the purpose of painting an overlay, is generic.

Change-Id: I6d3ee89cc51354988fedcc40340bb45a065db607
Reviewed-by: David Schulz <david.schulz@qt.io>

src/libs/utils/overlaywidget.cpp

@@ -0,0 +1,76 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 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.
+**
+****************************************************************************/
+
+#include "overlaywidget.h"
+
+#include "qtcassert.h"
+
+#include <QEvent>
+#include <QPainter>
+
+Utils::OverlayWidget::OverlayWidget(QWidget *parent)
+{
+    setAttribute(Qt::WA_TransparentForMouseEvents);
+    if (parent)
+        attachToWidget(parent);
+}
+
+void Utils::OverlayWidget::setPaintFunction(const Utils::OverlayWidget::PaintFunction &paint)
+{
+    m_paint = paint;
+}
+
+bool Utils::OverlayWidget::eventFilter(QObject *obj, QEvent *ev)
+{
+    if (obj == parent() && ev->type() == QEvent::Resize)
+        resizeToParent();
+    return QWidget::eventFilter(obj, ev);
+}
+
+void Utils::OverlayWidget::paintEvent(QPaintEvent *ev)
+{
+    if (m_paint) {
+        QPainter p(this);
+        m_paint(this, p, ev);
+    }
+}
+
+void Utils::OverlayWidget::attachToWidget(QWidget *parent)
+{
+    if (parentWidget())
+        parentWidget()->removeEventFilter(this);
+    setParent(parent);
+    if (parent) {
+        parent->installEventFilter(this);
+        resizeToParent();
+        raise();
+    }
+}
+
+void Utils::OverlayWidget::resizeToParent()
+{
+    QTC_ASSERT(parentWidget(), return );
+    setGeometry(QRect(QPoint(0, 0), parentWidget()->size()));
+}