Add CEGUI initialization

We now depend on CEGUI.
Starts implementing UIMain (incomplete)

Note : Compiling CEGUI on Windows/MSVC is a *pain*. The source code was
missing some #includes for algorithm.h (for std::min and std::max), and
the default generated project files were linking against the wrong Ogre
target (release instead of debug in debug mode), resulting in
hard-to-debug runtime crashes during CEGUI's initialization.