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.h

@@ -0,0 +1,55 @@
+/****************************************************************************
+**
+** 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.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "utils_global.h"
+
+#include <QWidget>
+
+namespace Utils {
+
+class QTCREATOR_UTILS_EXPORT OverlayWidget : public QWidget
+{
+    Q_OBJECT
+public:
+    using PaintFunction = std::function<void(QWidget *, QPainter &, QPaintEvent *)>;
+
+    explicit OverlayWidget(QWidget *parent = nullptr);
+
+    void attachToWidget(QWidget *parent);
+    void setPaintFunction(const PaintFunction &paint);
+
+protected:
+    bool eventFilter(QObject *obj, QEvent *ev) override;
+    void paintEvent(QPaintEvent *ev) override;
+
+private:
+    void resizeToParent();
+
+    PaintFunction m_paint;
+};
+
+} // namespace Utils