Add GRealClosure and move meta_marshallers there

Add GRealClosure and move meta_marshallers there

This means we're not abusing the notifiers for meta_marshallres,
and we're able to later cleanly add other fields to GClosure.

We still have to leave the ABI intact for the GClosure->meta_marshal
bit, as old G_CLOSURE_N_NOTIFIERS macro instances still accesses it.
However, we always set it to zero to keep those macros working.

https://bugzilla.gnome.org/show_bug.cgi?id=661140