Fix z-order when dragging a dock widget over another

the drop target would create a floating group window, which would
have higher z-order since it just had show() called on it.

raise the window we're dragging when the target is mutated into group
window.

Bug can be seen on the .gif attached to QTBUG-67611.

Change-Id: I5dad058468e24327b14d1e7f76c3ad0287d26ee8
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/widgets/qmainwindowlayout.cpp

@@ -2559,6 +2559,7 @@ void QMainWindowLayout::hover(QLayoutItem *widgetItem, const QPoint &mousePos)
                 dropTo->show();
                 dropTo->d_func()->plug(QRect());
                 w = floatingTabs;
+                widget->raise(); // raise, as our newly created drop target is now on top
             }
             Q_ASSERT(qobject_cast<QDockWidgetGroupWindow *>(w));
             auto group = static_cast<QDockWidgetGroupWindow *>(w);