The MetaKeyboardA11yFlags are used by gnome-shell to show a dialog
whenever a keyboard accessibility feature is switched using the
keyboard.
Unfortunately, commit c3acaeb25 renamed the Clutter flag to Meta and
moved them to a private header. As a result, gnome-shell do not show any
dialog anymore when a keyboard accessibility feature is activated.
Move the MetaKeyboardA11yFlags definition to a public header so that
gnome-shell can use it.
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2306
Fixes: c3acaeb25 - backends: Move keyboard a11y into backends
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2463>
It'll be part of and owned by MetaContext, intending to replace
`meta_is_wayland_compositor()`, but place it in a new file for public
enums so that it can be used from wherever.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>
