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