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 (see also my blog post about this [here](http://akzgsoc.blogspot.fi/2014/07/for-past-week-ive-been-working-on.html)):
- 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?
I know I'm saying a lot about comments on these, but I see now that there are a bunch of them. I still think it's prudent to note somewhere what has to have mutexes, and what not.