QDockWidget: call raise() when a dock widget starts to hover

When dock widget (1) starts to hover over another floating dock
widget (2), the latter animates a rubber band, to indicate to the user
that it is ready to accept a drop.

The creation of a QRubberBand moves (2) one position up in the Z order.
The consequence is a visual glitch: While
- the mouse cursor dragging (1) is still outside (2) and
- the visual rectangle of (1) starts overlapping (2)
(1) hides behind (2).
As soon as the mouse cursor enters (2), (1) suddenly comes on top and
(2) hides behind (1).

=> raise() 1 as soon as it starts hovering. That brings it on top of
the Z order, which is expected behavior.

Pick-to: 6.5
Change-Id: I1140fc6ff109c7a713e7e2617072698467375585
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: David Faure <david.faure@kdab.com>
(cherry picked from commit 1ab91b7bdbcced6ea4cc176da1d18e6b789208ae)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qmainwindowlayout.cpp

@@ -2791,6 +2791,7 @@ void QMainWindowLayout::hover(QLayoutItem *hoverTarget,
   QWidget *widget = hoverTarget->widget();
 
 #if QT_CONFIG(dockwidget)
+    widget->raise();
     if ((dockOptions & QMainWindow::GroupedDragging) && (qobject_cast<QDockWidget*>(widget)
             || qobject_cast<QDockWidgetGroupWindow *>(widget))) {