add support for Aliens-Versus-Predator

Another FPS from 2000

I don't know yet which fork of the engine (all based on https://icculus.org/avp/) is the best choice, but they all seems to lack the ability to play the Bink videos (this is why those files are marked 'optional'). Anyway, i'm not sure I will do the packaging of it, the "serioussam experiment" isn't really what we can call a success...