extensionSystem: Implement new live enable/disable system

The rough sketches of the system are outlined here:

http://mail.gnome.org/archives/gnome-shell-list/2011-June/msg00283.html

Additionally, enable/disable extensions when the 'enabled-extensions' setting
changes. Two new DBus methods, "EnableExtension" and "DisableExtension" allow
users to manipulate this setting quite easily.

https://bugzilla.gnome.org/show_bug.cgi?id=654770