EMI: Fix unstoppable sound

- if a pre-loaded sound is played again once the previous one is still
  playing, it can't be stopped
- there is only one sound handle, but the mixer would create two
  channels (indexed by the sound handle)
- so the channel created first could not be addressed any more
- stopping the sound (per handle) would not work

current work around: don't start one handle twice

other possible solution: make sure the mixer finds all channels which
play one handle (may require changes to common scummvm code)
EMI: Fix MP3Track::isPlaying()

- previous implementation always returned true
- check whether the mixer is actually playing the track
EMI: Fix VimaTrack::isPlaying()
EMI: Stop all PoolSounds when changing the set

- this fixes the problem that the fish sound was continuing
even after leaving the water without catching any of the fish
EMI: Fix format string of debug output
EMI: Fix costume handling

If a costume is not found in the existing costume stack of an actor,
try to load it via actor->pushCostume().
Merge pull request #777 from chkr-private/costume-and-chore-sound-fixes

EMI: various costume and sound fixes