Ensure the user gets notified when using too new versions of CEGUI and SDL.

We do not support SDL 2 or CEGUI 0.8 yet. Not enough resources.
Switch to libxml++.

This reverts 2682f5d9612e5c22b3cb6c9f349383e47f4d440c. Bare libxml2 does
not necessarily have a nice API, so we’re going with libxml++ instead.
Start extracting the XML loading process into a separate class
Adapt level background loading to new XML API
Fix missing int conversion
Centralize inclusion of library headers.

Including the library headers here and there cluttered all over the
source hinders getting a proper overview of what’s included. Even
worse, some headers need #ifdefs or even #undefs in front of them
which would have to be repeated for each file using this headers
over and over. So instead, we place all external includes into a
single file that can be included by any sourcefile to get immediate
access to all libraries we’re using. This has not been tested with
MinGW yet.

I’ve not yet removed the external includes from all the sourcefiles,
simply because I’ve been too lazy. I will do this as a side effect
in upcomming commits I guess.
Fix some obvious errors