backends: Introduce MetaIdleManager

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>

src/meta/meta-backend.h

@@ -29,6 +29,7 @@
 
 #include "clutter/clutter.h"
 #include "meta/meta-dnd.h"
+#include "meta/meta-idle-monitor.h"
 #include "meta/meta-remote-access-controller.h"
 
 #define META_TYPE_BACKEND (meta_backend_get_type ())
@@ -61,6 +62,9 @@ MetaDnd      *meta_backend_get_dnd   (MetaBackend *backend);
 META_EXPORT
 MetaSettings *meta_backend_get_settings (MetaBackend *backend);
 
+META_EXPORT
+MetaIdleMonitor * meta_backend_get_core_idle_monitor (MetaBackend *backend);
+
 META_EXPORT
 MetaRemoteAccessController * meta_backend_get_remote_access_controller (MetaBackend *backend);
 

src/meta/meta-idle-monitor.h

@@ -40,9 +40,6 @@ typedef void (*MetaIdleMonitorWatchFunc) (MetaIdleMonitor *monitor,
                                           guint            watch_id,
                                           gpointer         user_data);
 
-META_EXPORT
-MetaIdleMonitor *meta_idle_monitor_get_core (void);
-
 META_EXPORT
 guint         meta_idle_monitor_add_idle_watch        (MetaIdleMonitor          *monitor,
 						       guint64                   interval_msec,