backends: Centralize panel autorotation in a DisplayConfig property

Instead of having everyone check net.hadess.SensorProxy themselves, have
this all controlled by the MetaOrientationManager, and proxied everywhere
else via a readonly property in org.gnome.Mutter.DisplayConfig.

We want to attach more complex policies here, and it seems better to
centralize the handling of the autorotation feature rather than
implementing policy changes all over the place.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1311

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

@@ -129,6 +129,8 @@ struct _MetaMonitorManager
 
   int persistent_timeout_id;
 
+  guint panel_orientation_managed : 1;
+
   MetaMonitorConfigManager *config_manager;
 
   GnomePnpIds *pnp_ids;