This object takes over the functionality of meta-idle-monitor-dbus.c,
meta-idle-monitor.c and meta-backend.c, all related to higher level
management of idle watches etc.
The idle D-Bus API is changed to be initialized by the backend instead
of MetaDisplay, as it's more of a backend functionality than what
MetaDisplay usually deals with.
It also takes over the work of implementing "core" idle monitors. The
singleton API is replaced with thin wrapper functions on the backend.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1859>
