Use more forward declarations again, given header moc files now included
Commit 700a7de8 added some includes as needed for property types used in headers whose moc files were compiled separately in mocs_compilation.cpp. And follow-up commit d8c5b4a0 cleaned up now duplicated includes in the sources.
With the moc file included by the corresponding source file that is no longer needed. So this restores the old forward declarations, with some adaptions to additions meanwhile.