Add a new MusicFinished user event.

  This is usually better than polling, and more adequate to expected
event driving usage of the sdl interface.
Add basic support for sound effects.

  This adds the base types and simplest builtins to load, play and
release  SDL_Chunk objects.
Export almost every possible extra SDL2_mixer interface.

  This may still be redesigned a bit for better type checking, e.g. to
help users from causing errors due to assigning incorrect values to
channels.
  A new custom event type was also added, to generate an event when
a channel has finished playing. The new event returns the channel
identifier in the new channel field of event_t, and as the music one,
is a helper to avoid needing to poll to know when playing has finished.