Add extensionSystem

Consumer documentation will live at http://live.gnome.org/GnomeShell/Extensions

In terms of implementation; basically we load extensions from the well-known
directories.  Add a GConf key to disable extensions by uuid.  There is a new
option --create-extension for the gnome-shell script which takes a bit of
interactive input, sets up some sample files, and launches gedit.

No extensions UI in this patch; that will come later.

https://bugzilla.gnome.org/show_bug.cgi?id=599661

js/ui/main.js

@@ -14,6 +14,7 @@ const St = imports.gi.St;
 
 const Chrome = imports.ui.chrome;
 const Environment = imports.ui.environment;
+const ExtensionSystem = imports.ui.extensionSystem;
 const Overview = imports.ui.overview;
 const Panel = imports.ui.panel;
 const PlaceDisplay = imports.ui.placeDisplay;
@@ -129,6 +130,9 @@ function start() {
 
     _relayout();
 
+    ExtensionSystem.init();
+    ExtensionSystem.loadExtensions();
+
     panel.startupAnimation();
 
     let display = global.screen.get_display();