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.