Edit functions in multitrack

Added to multitrack class corresponding functions
InsertEvent(), InsertNote(), DeleteEvent(), DeleteNote().
Added also function IsValidTrackNum()
Updated documentation in msg.h and track.h

Added doxygen comments to these files, included new functions added by
me.
Documentation in msg.h yet unfinished
The method MIDITimedBigMessage::GetPan() was'n t declared in the header:
fixed and added a const specifier to the .cpp