Allow individual velocity/pan changes with alt

This commit changes the behavior introduced in 6e3d4f4 to allow using
alt to change the velocity of multiple *selected* notes by dragging.

I believe this was the originally intended behavior, but it never worked for me.

src/gui/editors/PianoRoll.cpp

@@ -2116,9 +2116,14 @@ void PianoRoll::mouseMoveEvent( QMouseEvent * me )
  bool isUnderPosition = n->withinRange( ticks_start, ticks_end );
  // Play note under the cursor
  if ( isUnderPosition ) { testPlayNote( n ); }
- // If note is the one under the cursor or is selected when alt is
- // not pressed
- if ( ( isUnderPosition && !isSelection() ) || ( n->selected() && !altPressed ) )
+ // If note is:
+ // Under the cursor, when there is no selection
+ // Selected, and alt is not pressed
+ // Under the cursor, selected, and alt is pressed
+ if ( ( isUnderPosition && !isSelection() ) || \
+ ( n->selected() && !altPressed ) || \
+ ( isUnderPosition && n->selected() && altPressed ) \
+ )
  {
  if( m_noteEditMode == NoteEditVolume )
  {