SCUMM: Fix typo in AD code.
SCUMM: Simplify voice channel allocation in AD code.
SCUMM: Only forward declare Serializer in music.h.
SCUMM: Slightly refactor music playback in AD player.
SCUMM: Implement simple seeking in AD player.
SCUMM: Save/load music/sfx data in AD player.

This makes sure that the currently playing music and sfx are resumed when
loading a save game.