Add lua api for the new music system

[Kolfering]

This PR basically adds 2 subtables to the Music lua table, Sequence & Segment. It also adds a new MusicFadeType enumeration to provide different fading functions even if for now only 2 are available (or 3 if we count the seamless transition)

Here is a music plugin using the new multiple lua plugins feature to demonstrate a simple case with the new api in use and how it behaves in game. Which will probably be easier to review than the PR to see how it works.

In it, only one music channel is used with 4 sequences:

• one sequence for the "normal track" composed of a single segment (default or triggered when the last time the player created a projectile was some time ago)
• one sequence for the "fight track" composed of 2 segments (triggered when the player creates projectiles)
• one sequence for the "hard fight track" of a single segment (triggered when the player creates projectiles and with hp < 75)
• one sequence for the "dead track" composed of a single segment that doesn't loop (triggered when the player hp <= 0)

All segment transitions are seamless transitions except the ones that lead to the dead sequence which use crossfades transition.
SegmentedMusic.zip

I also updated the xslt because I wanted the "new" functions for the subtables to be placed first in the documentation and sorting by name as it does now, was preventing that.