libgimp is anyway processed at the very end after all other libgimp*
were built. This way, it also fixes#3746, by removing the $(top_srcdir)
everywhere from introspected files, hence making the build work again
with older automake.
which means that it's now included normally via gimpbase.h
and not any longer via gimpbasetypes.h which we only did out
of lazyness. A *lot* of files in libgimp* and app/ now need to
all the stuff from app/core/gimpparamspecs.[ch] that is not about
image, drawable etc IDs, these will have to go to libgimp with
different implementations than in app/.
At first I thought these could be different namespaces, but actually
GObject Introspection parses files and can only use (AFAICS) the
namespace actually used in our C function, which is always `gimp_` (and
not `gimpbase_` or whatever.
So make the introspection at the root level, and it will include all
libgimp* libraries in one namespace, same as the C lib anyway. For now
only libgimp and libgimpbase as I am still testing.
Also I move the introspectable sources in their own file in order to
share the list between the library building Makefile and the GI
makefile because I can't find how to pass over variables otherwise.
Jehan