Fix stuck notes on notestacking

Fixes #865. Added a check so we don't add subnotes to a note that is already released (can happen with very short midinotes, eg. sliding the mouse accross the gui piano).
I also added a mutex to incoming midievent handling in instrumenttrack. Not sure if strictly necessary, but seems like this would prevent problems in the long run.
Also fixed a glitch in noteplayhandle where silent master notes got stuck playing indefinitely ( isMasternote() returns true even if the nph just hadChildren, so it never gets ended if it did - instead we check if we still have subnotes).

include/InstrumentTrack.h

@@ -230,6 +230,7 @@ private:
 	NotePlayHandle* m_notes[NumKeys];
 	int m_runningMidiNotes[NumKeys];
 	bool m_sustainPedalPressed;
+	QMutex m_notesMutex;
 
 	bool m_silentBuffersProcessed;