Update the messaging protocol spec to accomodate more reasonable file
loading for audio items.

Once again, this is just smart architecture paying off in an incomparably
stable DAW...  Instead of allowing the UI unrestricted access to the
back-ends memory space to wreak havoc and cause bugs+crashes, and then
using gargantuan Qt generic collections for storing audio path floats,
PyDAW maintains 100% separation of the 2, and the audio/MIDI bits are
written in very low-level C, for optimal performance and memory use,
without allowing bugs in the Qt framework to screw up anything...

Kind of like an MVC architecture for DAWs, for the same reason that
websites use MVC:  do you want things your UI/graphics people do to
have any chance of breaking the underlying web application?  Of course
not...
Fixes to audio tracks...  Still a few more fixes forthcoming when I wrap up some other work...
Fundamental re-working of the audio item loading to permit thread-safe loading/unloading of items...  Worked when I tried it, but needs more testing...