These are now referenced on the frames client side (in order to
track deletable state from the client window) and the mutter side
(pretty much everything else, like figuring out if a window wants
WM decorations).
It makes sense to make this a separate header, so that we don't
need to doubly define these flags/structs.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2735>
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