Deactivate Qt5Frame's reimplementation of Flush()

it causes too many issues: giant tooltips, dialogs can't be smaller
than 640x480 etc. Need to figure out a better solution

Change-Id: I4cc404687a60f2e0a39fd748fcc8cfea7cb7c0ac
Reviewed-on: https://gerrit.libreoffice.org/60038
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

vcl/qt5/Qt5Frame.cxx

@@ -602,7 +602,8 @@ void Qt5Frame::SetPointerPos(long nX, long nY)
 
 void Qt5Frame::Flush()
 {
-    QGuiApplication::sync();
+    // was: QGuiApplication::sync();
+    // but FIXME it causes too many issues, figure out sth better
 
     // unclear if we need to also flush cairo surface - gtk3 backend
     // does not do it. QPainter in Qt5Widget::paintEvent() is