st: Make BoxLayout use a Clutter layout manager internally

st: Make BoxLayout use a Clutter layout manager internally

With the BoxLayout containers in St and Mx and the ClutterBoxLayout
manager, we now have three more or less diverged implementations of
the same layout policy.

While removing StBoxLayout entirely in favor of ClutterLayoutManager
would be the fashionable thing to do, there are obvious drawbacks:

 - it is the only actor we have that implements the scrollable interface
 - it conveniently exposes its spacing property in CSS
 - last but not least, it is used all over the place

So do the next best thing and make our implementation use the
Clutter layout manager internally - that way, the change is
transparent to users, while we get to refer most of the tricky
bits to Clutter. win-win!

https://bugzilla.gnome.org/show_bug.cgi?id=703810