- QMouseEvent::screenPos() should be global desktop position
- QDesktopWidget::screenNumber() should tell the correct screen
- QGuiApplication::topLevelAt(screenPos) should find the window where
  the mouse is clicked

Change-Id: I9a63ab3ee1944b7246551d0f3d5e37f0d2aa5457
Reviewed-by: Błażej Szczygieł <spaz16@wp.pl>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>