Limit note length to quantization value

Draging a note to it's minimum value of 1 will add this new length to
the note if you later choose to stretch it which will not be clearly
visible in the Piano Roll unless you zoom in a bit. Limit the note
length to the quantization value and use <Alt> key to override and set
a smaller value.

src/gui/editors/PianoRoll.cpp

@@ -2811,8 +2811,8 @@ void PianoRoll::dragNotes( int x, int y, bool alt, bool shift, bool ctrl )
 				if (note->selected())
 				{
 					int newLength = note->oldLength() + off_ticks;
-					newLength = qMax(1, newLength);
-					note->setLength( MidiTime(newLength) );
+					newLength = qMax(alt ? 1 : quantization(), newLength);
+					note->setLength(MidiTime(newLength));
 
 					m_lenOfNewNotes = note->length();
 				}