Added a new SoundManager based on OpenAL. I also simplified the API along the

Added a new SoundManager based on OpenAL. I also simplified the API along the
way.
OpenAL gives you some neat effects like positional sounds moving along while
they are played. Try a level with a flame to see this in action.

Some notes:
- don't forget to run autogen.sh and configure again
- The AmbientSound object probably needs an overhaul now
- You can't play .mod files anymore, they have to be converted to .ogg
- There's a bug that keeps supertux open for several seconds instead of closing
  it, didn't investigate yet why.

SVN-Revision: 2636