DisplayConfig: Add 'supports-changing-layout-mode' property

This property may be used by a client to determine whether it is
allowed to change the layout mode.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-monitor-manager.c

@@ -1531,11 +1531,14 @@ meta_monitor_manager_handle_get_current_state (MetaDBusDisplayConfig *skeleton,
                              g_variant_new_boolean (FALSE));
     }
 
+  g_variant_builder_add (&properties_builder, "{sv}",
+                         "layout-mode",
+                         g_variant_new_uint32 (manager->layout_mode));
   if (capabilities & META_MONITOR_MANAGER_CAPABILITY_LAYOUT_MODE)
     {
       g_variant_builder_add (&properties_builder, "{sv}",
-                             "layout-mode",
-                             g_variant_new_uint32 (manager->layout_mode));
+                             "supports-changing-layout-mode",
+                             g_variant_new_boolean (TRUE));
     }
 
   if (meta_monitor_manager_get_max_screen_size (manager,

src/org.gnome.Mutter.DisplayConfig.xml

@@ -398,6 +398,9 @@
 			     using the logical monitor scale.
 	    * 2 : physical - the dimension of a logical monitor is derived from
 			     the monitor modes associated with it.
+	* "supports-changing-layout-mode" (b): True if the layout mode can be
+					       changed. Absence of this means the
+					       layout mode cannot be changed.
     -->
     <method name="GetCurrentState">
       <arg name="serial" direction="out" type="u" />