Audio: Remove hardcoded sounds

Loading sounds is now a job for game itself, not the hrengin.
Audio: Implement ISound class

I cleaned up audio code, now sound has it's own class, buffers are now
stored it CSoundManager::SoundMap_, so they aren't recreated each time.
As always, error handling is not present yet.

Also I renamed ::hrengin::sound namespace to ::hrengin::audio, to match
the module name.
Audio: Add todolist

Added todolist file, and some future stuff.