NotePlayHandle: emit midiNoteOff() signal in noteOff()

Instead of emitting InstrumentTrack::midiNoteOff() in destructor of
NotePlayHandle do this where it actually happens -> noteOff().

Fixes length of recorded notes when there's e.g. a long release.

Closes #378.
NotePlayHandle: coding style fixes
Removed traces of old SurroundArea implementation
Note, NotePlayHandle: no default arguments for setVolume() and setPanning()
Note: use qBound() instead of tLimit()