* Add new trigger types: raise, fall
* Add new trigger offset parameter
* Add new volume/pitch types: inv, abs, sqrt
* Add initial support for multiple events to intervere in a single sound
(by using the same name, see the crank section of 172-sound.xml)
* Cached the volume/pitch type fucntions
(No more if's inside the loops in update())
individual aircraft to have different sounds (and cleaning up my code
a fair bit). The most important user-visible change is the renaming
of the /sim/sound property to /sim/sound/audible.