SCUMM: Small cleanup in SFX note setup in AD code.
SCUMM: Slight cleanup in SFX register code in AD player.
SCUMM: Introduce symbolic names for channel/note states in AD code.
SCUMM: Only keep music data variable for music around in AD code.
SCUMM: Further SFX note related cleanup in AD player.
SCUMM: Move notes into SFX channel structure in AD player.
SCUMM: Fix sfx resource unlocking in AD player.
SCUMM: Replace some more constants with named enum values in AD code.
SCUMM: Store hardware channel number explicitly in Channel structure in AD code.
SCUMM: Always work directly on Channel data in AD code.
SCUMM: Further refactor AD SFX code.
SCUMM: Further cleanup in AD SFX code.
SCUMM: Properly unlock sound resources when reusing sfx slots in AD code.
SCUMM: Clean up how SFX are stopped in the AD player.
SCUMM: Allow multiple SFX to be played at once with AD.

Actually, even before this change it was possible. However, since we formerly
used the original channel specification it was not possible when two sfx were
started on the same channel. Now we can play such sounds simultaneously again.
This is a first step towards a AdLib support level we had when we tried to
convert SFX to MIDI to play it through iMuse. However, now we still play the
sound effects properly, i.e. they still sound like in the original.
SCUMM: Dynamically allocate hw channels for SFX in AD player.
SCUMM: Let music in AD code use the same HW channel allocation as SFX.
SCUMM: Only stop music when requested in AD code.
SCUMM: Allow music and sfx to be played at the same time in AD player.
SCUMM: Only reset rhythm state when music is stopped.