Call finishDrag() in QWaylandDataDevice::dragSourceCancelled()

Drags can either get finished or cancelled. If a drag is finished
successfully we call finish on the QBasicDrag instance, which quits
the nested event loop. This patch adds the connection for cancelled
drags.

See also: https://bugs.kde.org/show_bug.cgi?id=446111

Pick-to: 6.4 6.2 5.15
Change-Id: Ib93040648da88a433d647c87adcb7a7fabcaef6c
Reviewed-by: Liang Qi <liang.qi@qt.io>

src/plugins/platforms/wayland/qwaylanddatadevice.cpp

@@ -261,6 +261,7 @@ void QWaylandDataDevice::selectionSourceCancelled()
 #if QT_CONFIG(draganddrop)
 void QWaylandDataDevice::dragSourceCancelled()
 {
+    static_cast<QWaylandDrag *>(QGuiApplicationPrivate::platformIntegration()->drag())->finishDrag();
     m_dragSource.reset();
 }