codegen: Install Python files in $(datadir)/glib-2.0, rather than $(libdir)

These files are actually architecture-indepdendent; using $(libdir)
for them means that /usr/bin/gdbus-codegen varies between
architectures, which is problematic for (mis)uses of multilib.

See https://bugzilla.redhat.com/show_bug.cgi?id=718404

https://bugzilla.gnome.org/show_bug.cgi?id=685012