GUI overhaul

Removed LibRocket and integrated CEGUI as my GUI library. The combat log
is now (finally) implemented and functioning almost as intended
(formatted, multi-coloured messages such as "X hit Y for Z" aren't yet
implemented, but the performance issues plaguing all LibRocket-based
implementations are gone).

Next step after finishing the combat log is to re-implement the party-
and abilities bar using CEGUI before the GUI is finished.