small fixes, rewrite SoundManager to use static methods

new API for SoundManager:
   SoundManager& instance()

  Sound:
   static bool registerSound(path, name)
   static void playSound(name, volume=100.0f)
   static void cleanQueue() -- call this every once in a while

  Music:
   static bool registerMusic(path, name)
   static sf::Music* getMusic(name)

  all other stuff is either removed or private

small fixes:
  main loads sounds using static methods
  collectibles are on layer 4
  bobms use playSound for explosions