backend: Add 'experimental-features' gsetting

This gsetting will allow the adding of keywords to a array, where each
keyword may enable an experimental feauter, if the given mutter version
supports that particular experimental feature. Emphasis is put on the
lack of guarantee that any such keyword has any effect. Currently no
keywords are defined.

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

data/org.gnome.mutter.gschema.xml.in

@@ -102,6 +102,20 @@
       </description>
     </key>
 
+    <key name="experimental-features" type="as">
+      <default>[]</default>
+      <summary>Enable experimental features</summary>
+      <description>
+        To enable experimental features, add the feature keyword to the list.
+        Whether the feature requires restarting the compositor depends on the
+        given feature. Any experimental feature is not required to still be
+        available, or configurable. Don't expect adding anything in this
+        setting to be future proof.
+
+        Currently possible keywords: (none)
+      </description>
+    </key>
+
     <child name="keybindings" schema="org.gnome.mutter.keybindings"/>
 
   </schema>