Qt: fixed crash in TQtWidgetSet.MessageBox if passed HWND is 0.


git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@41827 4005530d-fff6-0310-9dd1-cebe43e6787f
gtk3: implemented TGtk3WidgetSet.MessageBox


git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@41828 4005530d-fff6-0310-9dd1-cebe43e6787f