MetaGravity is an enum, where the values match the X11 macros used for
gravity, with the exception that `ForgetGravity` was renamed
`META_GRAVITY_NONE` to have less of a obscure name.
The motivation for this is to rely less on libX11 data types and macros
in generic code.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/705
The order and way include macros were structured was chaotic, with no
real common thread between files. Try to tidy up the mess with some
common scheme, to make things look less messy.
