QGraphicsWidget: add missing break statement to switch in event()

If the QEvent::GraphicsSceneMousePress case falls through, it does so
because d->hasDecoration() == false or the virtual call to
windowFrameEvent() returned false.

It falls through to the case for QEvent::GraphicsSceneMouseMove, etc,
which ensures d->windowData and then checks hasDecoration() again,
with some other conditions on top, and calls the same virtual
function, windowFrameEvent(), with the same arguments again.

Now, it could, theoretically, be possible that that second call would,
due to the presence of a windowData that wasn't there before, return
true when before it did return false. But the only modification to
*this between the calls to windowFrameEvent() is the potential
allocation of d->windowData, which, if actually effected, will have
d->windowData->grabbedSection == Qt::NoSection, hence
windowFrameEvent() won't even be called a second time

It is therefore safe to assume that a break was intended here, so add
it.

Discovered independently be GCC 7 and Coverity.

Coverity-Id: 11149
Change-Id: Id708a1689ed0f0c914622e388c456ea4576fda02
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/widgets/graphicsview/qgraphicswidget.cpp

@@ -1449,6 +1449,7 @@ bool QGraphicsWidget::event(QEvent *event)
     case QEvent::GraphicsSceneMousePress:
         if (d->hasDecoration() && windowFrameEvent(event))
             return true;
+        break;
     case QEvent::GraphicsSceneMouseMove:
     case QEvent::GraphicsSceneMouseRelease:
     case QEvent::GraphicsSceneMouseDoubleClick: