LCL: gtk2: dropfiles for sub forms

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@45559 4005530d-fff6-0310-9dd1-cebe43e6787f
LCL: qt, gtk1, win32: dropfiles for sub forms

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@45560 4005530d-fff6-0310-9dd1-cebe43e6787f