Separate MonoBook skin from core

Separate MonoBook skin from core

This makes it behave exactly like a custom skin.

* Renamed directory to reflect skin name.
* Split skin classes to separate PHP files.
* Removed core autoloader entries for skin classes.
* Changed the hack in Setup.php to require_once the skin PHP file, as
  the skin is now registered there.
* Extracted skin-specific localisation messages.
* Extracted skin-specific resources.

Change-Id: Ife9926d12b6baaa84cd2aa9a415f1183415863c8