Fix crash when hosting QQuickWidget

There's no need to check if invisible widgets are candidates
for docking. There's usually no downside either, except that
QQuickWidget is very sensitive with when its platform window
is created, so don't force the creation.

Will cherry-pick to 1.2 if no regression is found

Fixes #150

src/private/DragController.cpp

@@ -648,6 +648,10 @@ static QWidgetOrQuick *qtTopLevelForHWND(HWND hwnd)
 {
     const QList<QWindow*> windows = qApp->topLevelWindows();
     for (QWindow *window : windows) {
+        if (!window->isVisible()) {
+            continue;
+        }
+
         if (hwnd == (HWND)window->winId()) {
             return DockRegistry::self()->topLevelForHandle(window);
         }