it was not a step in the right direction in 1st place. We distinguish
between main window frame (QMainWindow) and everything else (QWidget)
differently now

Change-Id: I331c9760f7616b2f199e63e5e521df74b08acc62
Reviewed-on: https://gerrit.libreoffice.org/58853
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>