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/backends/meta-backend.c

@@ -430,6 +430,8 @@ experimental_features_handler (GVariant *features_variant,
       /* So far no experimental features defined. */
       if (g_str_equal (feature, "scale-monitor-framebuffer"))
         features |= META_EXPERIMENTAL_FEATURE_SCALE_MONITOR_FRAMEBUFFER;
+      else if (g_str_equal (feature, "monitor-config-manager"))
+        features |= META_EXPERIMENTAL_FEATURE_MONITOR_CONFIG_MANAGER;
       else
         g_info ("Unknown experimental feature '%s'\n", feature);
     }