lcl: fix position of poDesktopCenter forms when desktop Left,Top <> (0,0)

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42098 4005530d-fff6-0310-9dd1-cebe43e6787f
qt: fix DesktopLeft and DesktopTop requests

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42099 4005530d-fff6-0310-9dd1-cebe43e6787f