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.