EMI: Improvements to the sound system

This is a collection of fixes and improvements to the sound system in EMI.

The most notable changes are the following improvements to music playback:
- Cue point data from .jmm files is now respected, so music tracks will start at the correct position and loop the correct sub-section of the track
- Music syncing is implemented (noticeable for example when entering the fountain closeup in Melee town)
- When the music track changes, the music now transitions with a cross-fade

The sounds are now also saved and restored properly.

f391e57 contains a fix for an apparent bug in common MP3 streaming code, which causes `MP3Stream`'s seek method to not behave as expected when seeking backwards. This should perhaps be sent to ScummVM for review as well?