Mark QApplication::globalStrut as deprecated

This property is obsolete with today's user interface reality, and was
never consistently implemented in all styles and widgets. Deprecating
it for Qt 5.15 as preparating for removing the code for Qt 6.

[ChangeLog][QtWidgets][QApplication] The globalStrut property has been
deprecated and will be removed from Qt 6

Change-Id: Iaea88e563144481a395de6cf0745a512e386407d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/gui/doc/snippets/code/src_gui_kernel_qapplication.cpp

@@ -91,7 +91,7 @@ int main(int argc, char *argv[])
 //! [3]
 QSize MyWidget::sizeHint() const
 {
-    return QSize(80, 25).expandedTo(QApplication::globalStrut());
+    return QSize(80, 25);
 }
 //! [3]
 

src/widgets/doc/snippets/code/src_gui_kernel_qapplication.cpp

@@ -93,7 +93,7 @@ int main(int argc, char *argv[])
 //! [3]
 QSize MyWidget::sizeHint() const
 {
-    return QSize(80, 25).expandedTo(QApplication::globalStrut());
+    return QSize(80, 25);
 }
 //! [3]
 

src/widgets/kernel/qapplication.cpp

@@ -1224,6 +1224,7 @@ void QApplication::setColorSpec(int spec)
     \property QApplication::globalStrut
     \brief the minimum size that any GUI element that the user can interact
            with should have
+    \deprecated
 
     For example, no button should be resized to be smaller than the global
     strut size. The strut size should be considered when reimplementing GUI

src/widgets/kernel/qapplication.h

@@ -79,7 +79,9 @@ class Q_WIDGETS_EXPORT QApplication : public QGuiApplication
 #if QT_CONFIG(wheelevent)
     Q_PROPERTY(int wheelScrollLines  READ wheelScrollLines WRITE setWheelScrollLines)
 #endif
+#if QT_DEPRECATED_SINCE(5, 15)
     Q_PROPERTY(QSize globalStrut READ globalStrut WRITE setGlobalStrut)
+#endif
     Q_PROPERTY(int startDragTime  READ startDragTime WRITE setStartDragTime)
     Q_PROPERTY(int startDragDistance  READ startDragDistance WRITE setStartDragDistance)
 #ifndef QT_NO_STYLE_STYLESHEET
@@ -158,8 +160,10 @@ public:
     static void setWheelScrollLines(int);
     static int wheelScrollLines();
 #endif
+#if QT_DEPRECATED_SINCE(5, 15)
     static void setGlobalStrut(const QSize &);
     static QSize globalStrut();
+#endif
 
     static void setStartDragTime(int ms);
     static int startDragTime();