One can add a wl_global filter to a wl_display instance, which can be
used to decide what clients should see what globals. This has so far
been used to limit a Xwayland specific protocol extension to only
Xwayland. In order to expand the logic about what globals are filtered
to what clients, introduce a filter manager and port the Xwayland
specific protocol filter to this new manager.
Tests are added, using a new dummy protocol, to ensure that filtering is
working as expected.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2810>