PianoRoll: allow chaning volume and panning of step notes

There's no reason to not allow changing volume and panning of step notes
in PianoRoll. Therefore simply check whether the length is not equal to
zero (which might be disabled step notes) instead of checking for length
being greater than zero.

Closes #2836799.
(cherry picked from commit 8286c1031d)

src/gui/piano_roll.cpp

@@ -2014,7 +2014,7 @@ void pianoRoll::mouseMoveEvent( QMouseEvent * _me )
 			{
 				if( ( *it )->pos().getTicks() >= ticks_start 
 					&& ( *it )->pos().getTicks() <= ticks_end
-					&& ( *it )->length().getTicks() > 0
+					&& ( *it )->length().getTicks() != 0
 					&& ( ( *it )->selected() || ! use_selection ) )
 				{
 					m_pattern->dataChanged();