[fluid-dev] Lots of API doc updates

[josh]

Just committed the bulk of the remaining API doc updates.  The Doxygen  
documentation is now fairly complete.  Just need to figure out why  
lots of the internal files are getting included in the files list and  
also make sure that any undocumented elements get reported when  
Doxygen is run.

I also made some minor changes to the API:
Added correct spelling of FLUID_SEQ_PITCHWHEELSENS
Added FLUID_OK and FLUID_FAILED to public API
Renamed fluid_sequencer_get_useSystemTimer() to  
fluid_sequencer_get_use_system_timer() to be more consistent.

I was looking at some of the public voice API functions and think that  
some of them need a little work to make them thread safe, since it  
seems desirable for some of them to be usable outside of synth context.

Regards,
Josh