extensionSystem: Add methods to enable/disable extensions

Extensions are currently enabled or disabled by directly changing the
list in the 'enabled-extensions' GSettings key. As we will soon add
an overriding 'disabled-extensions' key as well, it makes sense to
offer explicit API for enabling/disabling to avoid duplicating the
logic.

For the corresponding D-Bus API, the methods were even mentioned in
the GSettings schema, albeit unimplemented until now.

https://bugzilla.gnome.org/show_bug.cgi?id=789852

data/dbus-interfaces/org.gnome.Shell.Extensions.xml

@@ -173,6 +173,30 @@
       <arg type="s" direction="in" name="uuid"/>
     </method>
 
+    <!--
+        EnableExtension:
+        @uuid: The UUID of the extension
+        @success: Whether the operation was successful
+
+        Enable an extension.
+    -->
+    <method name="EnableExtension"> \
+      <arg type="s" direction="in" name="uuid"/> \
+      <arg type="b" direction="out" name="success"/> \
+    </method> \
+
+    <!--
+        DisableExtension:
+        @uuid: The UUID of the extension
+        @success: Whether the operation was successful
+
+        Disable an extension.
+    -->
+    <method name="DisableExtension"> \
+      <arg type="s" direction="in" name="uuid"/> \
+      <arg type="b" direction="out" name="success"/> \
+    </method> \
+
     <!--
         LaunchExtensionPrefs:
         @uuid: The UUID of the extension