Move pygi foreign API into pygi-foreign-api.h

Move limited set of APIs necessary for registering foreign marshalers into
pygi-foreign-api.h. Remove "_real" from internally used APIs and add necessary
includes to the rest of pygobject for calling directly (instead of going through
the PyCapsule API within PyGI itself).
This is needed to avoid compilation errors when including pygobject.h in
foreign marshaling plugins which conflicts with pygobject-private.h.

https://bugzilla.gnome.org/show_bug.cgi?id=694604