stylecontext: Deprecate regions
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.
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