Previously, gsd-color handled adding color devices. It got information
about those via the GnomeRR API, which is part of libgnome-desktop.
libgnome-desktop itself got this information from the
org.gnome.Mutter.DisplayConfig.GetResources() D-Bus method, implemented
by mutter.
Now, mutter itself will add all the monitor color devices itself,
without having to go via gsd-color.
We sometimes need to delete colord devices synchronously, in certain
race conditions when we add and remove devices very quickly (e.g. in
tests). However, we cannot use libcolord's 'sync' API variants, as it
has a nested takes-all main loop as a way to invoke the sync call. This
effectively means we end up sometimes not return from this function in a
timely manner, causing wierd issues.
Instead, create our own sync helper, that uses a separate context that
we temporarly push as the thread-default one.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2141>
