Add XDG WM Base support to our XDGShell API

Summary:
This adds XDG WM Base (essentially XDG Shell v7/stable edition) into our
existing XDGShell classes which wrap v5, v6 and now this.

It's mostly copy and paste from V6 except for the enum types for gravity
and anchor edges on positioners.

There's been no attempt to share code with V6 as realistically that
won't get updates whereas XDGWMBase will; and at some point we will
want to drop V6 without things being too tangled.

Test Plan:
Same test suite as V6 has

Compiled GTK master and ran against suitably modified kwin
running WAYLAND_DEBUG=1 gtk-demo showed we were using this interface
Everything worked as well as V6 does.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D13510