KYRA: FM-Towns audio driver rewrite

- FM-Towns euphony driver completely rewritten based on KYRA FM-Towns and LOOM towns disasm.

- Split all the emu and driver code from sound_towns.cpp into different files to make things a bit less confusing.

- Move the driver code to common space since the exact same euphony driver is used by LOOM which means we could get rid of the outdated and incomplete ym2612 driver/emu implementation (which doesn't even do things like instrument loading, pan position, etc). I haven't tried to add this to the Scumm engine yet, since I am not  familiar with it and my priority was to get the driver finished first. But from the look of disasm it shouldn't be difficult to do.

- Introduce a generic FM-Towns audio interface based on FM-Towns system file disasm which was necessary for the euphony driver rewrite. Every FM-Towns game I have seen so far seems to access the audio hardware via these system functions. This interface implementation will also allow reasonably simple creation of new FM-Towns audio drivers (e.g. this could be used for Kings Quest 5 FM-Towns or others).

- Move the PC98 driver to common space, too, since I have a strong feeling that this driver is also used in the PC98 version of Future Wars

- This also improves KYRA FM-Towns music quality, sound effects accuracy and music fading.

svn-id: r51645

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