Start script for conversion of levels to new format.

To make SMC thinner, I’m going to remove the backward compatibility
stuff on level loading. As a compromise to not make existing levels
unusable, I’m also writing a conversion script that transforms
existing .smclvl files into a format SMC 2 can understand. This way
the main C++ code can focus on the actual loading and does not have
to care about old versions of SMC (for now).
Add player tag parsing.

For now, we’ll leave the backward compatibility in. First we replace
CEGUI XML stuff with libxml++.
Add code for level object element parsing