rhbz#1431476 dnd has started copying instead of moving slides

Is suspect something has changed in gtk3, and I think we're
not supposed to use gdk_drag_drop_succeeded at this point, but
instead we'd get a dragFailed signal on the failure case

Change-Id: I6b305db56a29255b468ca9182d1f7e57cb4c10f4

vcl/unx/gtk3/gtk3gtkframe.cxx

@@ -4231,7 +4231,7 @@ void GtkDragSource::dragEnd(GdkDragContext* context)
     {
         datatransfer::dnd::DragSourceDropEvent aEv;
         aEv.DropAction = GdkToVcl(gdk_drag_context_get_selected_action(context));
-        aEv.DropSuccess = gdk_drag_drop_succeeded(context);
+        aEv.DropSuccess = true;
         auto xListener = m_xListener;
         m_xListener.clear();
         xListener->dragDropEnd(aEv);