Regions are done in a very non-css way. They don't fit the DOM in that
they don't integrate into the CSS tree and they have very weird matching
behavior in selectors.

So I'm deprecating them now. GtkNotebook and GtkTreeview will continue
to use them and as long as they do, we can't remove the code for it.
But once those are ported it might be safe to remove the code as it will
clean up lots of places in the code by quite a bit.