xavix progress - mostly sound

[mamehaze]

Attempted to improve XaviX audio emulation with hardware studies [ramacat]

• sample loop start positions are no longer being treated as left/right channel start positions
• stop conditions are longer inverted, key-off no longer being treated as stop etc.
• attempted to implement various envelope effects and more (some use wave in the ROM, others are fixed function)
• some things sound much better (taitons2 etc.) while at times some notes hang until a new sample is played in other games (popira for example)

Added Save State support to the XaviX sound hardware emulation [David Haywood]

Fixed some regressions with SEEPROM handling in the driver [David Haywood]

[mamehaze]

setting as draft, because while I've done a decent amount of cleanup on the submission, there are probably still things that could be improved.

[mamehaze]

(also I'm fine with some stuff being a bit worse for now, as some of the guesses I made when I first tried to work this out were terrible. this sets things up for progress in the right direction)