monitor-manager: Make the config manager an 'experimental feature'

Instead of using a environment variable, add a new 'experimental
feature' gsetting keyword "monitor-config-manager" that enables the use
of the new MetaMonitorConfigManager. This commit also makes it possible
to switch between the two systems without restarting mutter.

The D-Bus API is disabled when the experimental feature is not enabled,
and clients trying to access it will get a access-denied error in
response. A new property 'IsExperimentalApiEnabled' is added to let the
D-Bus client know whether it is possible to use the experimental API or
not.

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

src/tests/unit-tests.c

@@ -219,9 +219,15 @@ run_tests (gpointer data)
 
   meta_backend_override_experimental_features (backend);
 
-  meta_backend_enable_experimental_feature (
-    backend,
-    META_EXPERIMENTAL_FEATURE_SCALE_MONITOR_FRAMEBUFFER);
+  if (g_strcmp0 (g_getenv ("MUTTER_USE_CONFIG_MANAGER"), "1") == 0)
+    {
+      meta_backend_enable_experimental_feature (
+        backend,
+        META_EXPERIMENTAL_FEATURE_MONITOR_CONFIG_MANAGER);
+      meta_backend_enable_experimental_feature (
+        backend,
+        META_EXPERIMENTAL_FEATURE_SCALE_MONITOR_FRAMEBUFFER);
+    }
 
   ret = g_test_run ();