Implements StepNotes setting a NPH with 0 frames

	Now, instead of TimePos returning 0 for negative lengths, we
create a NotePlayHandle with 0 frames when the note type is StepNote on
InstrumentTrack::play.

src/core/TimePos.cpp

@@ -158,11 +158,11 @@ tick_t TimePos::getTickWithinBeat( const TimeSig &sig ) const
 
 f_cnt_t TimePos::frames( const float framesPerTick ) const
 {
-	if( m_ticks >= 0 )
-	{
-		return static_cast<f_cnt_t>( m_ticks * framesPerTick );
-	}
-	return 0;
+	//TODO: Remove next line, it's just a warning to see
+	// if any leftover code is still creating notes with
+	// negative lengths
+	if (m_ticks < 0) { qWarning("TimePos with negative length"); }
+	return static_cast<f_cnt_t>(m_ticks * framesPerTick);
 }
 
 double TimePos::getTimeInMilliseconds( bpm_t beatsPerMinute ) const

src/tracks/InstrumentTrack.cpp

@@ -723,8 +723,11 @@ bool InstrumentTrack::play( const TimePos & _start, const fpp_t _frames,
 		while( nit != notes.end() &&
 					( cur_note = *nit )->pos() == cur_start )
 		{
-			const f_cnt_t note_frames =
-				cur_note->length().frames( frames_per_tick );
+			// If the note is a Step Note, frames will be 0 so the NotePlayHandle
+			// plays for the whole length of the sample
+			const f_cnt_t note_frames = cur_note->type() == Note::StepNote
+				? 0
+				: cur_note->length().frames(frames_per_tick);
 
 			NotePlayHandle* notePlayHandle = NotePlayHandleManager::acquire( this, _offset, note_frames, *cur_note );
 			notePlayHandle->setBBTrack( bb_track );