monitor-manager: Clean up DPMS state tracking

DPMS is configured from a bit all over the place: via D-Bus, via X11 and
when reading the current KMS state. Each of these places did it slightly
differently, directly poking at the field in MetaMonitorManager.

To make things a bit more managable, move the field into a new
MetaMonitorManagerPrivate, and add helpers to get and set the current
value. Prior to this, there were for example situations where the DPMS
setting was changed, but without signal listeners being notified about
it.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/506

src/backends/meta-monitor-manager-private.h

@@ -116,8 +116,6 @@ struct _MetaMonitorManager
   gboolean in_init;
   unsigned int serial;
 
-  MetaPowerSave power_save_mode;
-
   MetaLogicalMonitorLayoutMode layout_mode;
 
   int screen_width;
@@ -192,6 +190,8 @@ struct _MetaMonitorManagerClass
   GBytes* (*read_edid) (MetaMonitorManager *,
                         MetaOutput         *);
 
+  void (*read_current_state) (MetaMonitorManager *);
+
   void (*ensure_initial_config) (MetaMonitorManager *);
 
   gboolean (*apply_monitors_config) (MetaMonitorManager      *,
@@ -305,6 +305,11 @@ void                meta_monitor_manager_get_screen_size   (MetaMonitorManager *
                                                             int                *width,
                                                             int                *height);
 
+MetaPowerSave       meta_monitor_manager_get_power_save_mode (MetaMonitorManager *manager);
+
+void                meta_monitor_manager_power_save_mode_changed (MetaMonitorManager *manager,
+                                                                  MetaPowerSave       mode);
+
 void                meta_monitor_manager_confirm_configuration (MetaMonitorManager *manager,
                                                                 gboolean            ok);