SDL2 support

This adds SDL2 support for the "emulator" variant. It will be enabled if an SDL2 library can be loaded, otherwise it will fall back to SDL1.2.

Not that we necessarily need SDL2 for the "emulator". But there's a nice thing about SDL2: It brings an Android backend. Probably a lot more things will have to be adapted in order to work on Android, but I would like to try out the SDL route.