Separate Vector skin from core

This makes it behave exactly like a custom skin, with the caveat that
it is still hardcoded in several places :(, most notably lots of
skinStyles in Resources.php, the installer and some tests.

* 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. Did not touch skinStyles yet.
* Hacked up the installer not to fall over entirely if Vector is
  missing.
* Adjusted hardcoded paths in some more places...

Change-Id: Idfffc1430790b3a104cc9835a6367137bcbf0e4e