fixes #4049: Off-grid nodes not draggable (#6852)

The quantPos arg of AutomationClip::setDragValue causes the node time
to be quantized before its looked up in the timeMap iterator.
This results in the node not being found and a new one being created
inside the setDragValue function even though we had found one.
Simply setting it to true causes this bug, simply setting it to false
causes a new node to be created off grid/not snapped. So the fix
is to quantize the position for the lookup only if we haven't found an
existing node under the cursor.

src/gui/editors/AutomationEditor.cpp

@@ -531,7 +531,7 @@ void AutomationEditor::mousePressEvent( QMouseEvent* mouseEvent )
 								: POS(clickedNode)
 							),
 							level,
-							true,
+							clickedNode == tm.end(),
 							mouseEvent->modifiers() & Qt::ControlModifier
 						);