Better minimum length when resizing notes (#5512)

* 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.

* Update src/gui/editors/PianoRoll.cpp

Co-authored-by: Spekular <Spekular@users.noreply.github.com>

* Remember min note length if shorter than quantization()

* Find note length modulo quantization, pick smallest from selected notes

* Comment on and improve m_minResizeLen calculation

Co-authored-by: Oskar Wallgren <oskar.wallgren13@gmail.com>

include/PianoRoll.h

@@ -406,6 +406,9 @@ private:
 	volume_t m_lastNoteVolume;
 	panning_t m_lastNotePanning;
 
+	//When resizing several notes, we want to calculate a common minimum length
+	MidiTime m_minResizeLen;
+
 	int m_startKey; // first key when drawing
 	int m_lastKey;