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 no be clearly
visible in the Piano Roll unless you zoom in a bit. Limit the note
length to the quantization value and use <shift> key to set a smaller
value.

src/gui/editors/PianoRoll.cpp

@@ -2566,7 +2566,7 @@ 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);
+					newLength = qMax( quantization(), newLength );
 					note->setLength( MidiTime(newLength) );
 
 					m_lenOfNewNotes = note->length();