Start removing #ifdef ENABLE_NEW_LOADER and make it the default.

From this commit on, the new XML handling will be the *only* one.
The CEGUI XML stuff will now be completely removed.
Add NotImplementedError
Typo
Remove dependency on CEGUI NULL renderer.

YES!!11eins!elf

We do not depend on CEGUI’s NullRenderer anymore. This has been
requested so often, and now we finally have it. Yeah!
Move editor core files into their own directory
Convert editor XML handling to libxml++.

UNTESTED.