Add GObject Introspection annotations to methods where needed, in
particular adding (transfer none) to return values that don't transfer
ownership.
st_texture_cache_get_actor() and st_texture_cache_get_texture()
are annotated as (transfer none) since they return a newly
created *floating* texture.
https://bugzilla.gnome.org/show_bug.cgi?id=591245
Import the core MxWidget/MxBin and their dependencies; we use the
namespace "St" (Shell Toolkit) because it is the same length as Mx
so enabling easy sharing of code, but makes it clear that this is
a friendly fork and not a literal import.
Based on a patch by Colin Walters <walters@verbum.org>
https://bugzilla.gnome.org/show_bug.cgi?id=591245
