Fix small bug on LMMS#5848 (LMMS#5933)

m_gridMode was not being initialized on the PianoRoll
constructor. For that reason the first note drag without changing the
GridMode would result in m_gridMode being used unintialized and neither
the Snap nor Nudge mode being used. Here, the result of this was a note
being moved unquantized.
	This fixes it by calling changeSnapMode() after setting up the
snapModel on the constructor.

src/gui/editors/PianoRoll.cpp

@@ -442,6 +442,7 @@ PianoRoll::PianoRoll() :
 	m_snapModel.addItem(tr("Nudge"));
 	m_snapModel.addItem(tr("Snap"));
 	m_snapModel.setValue(0);
+	changeSnapMode();
 	connect(&m_snapModel, SIGNAL(dataChanged()),
 		this, SLOT(changeSnapMode()));