- When clicking a stroke, they aren't moved immediately anymore; a
certain drag distance is necessary, which makes it easy (again) to
select a stroke with a stylus (which tends to move a little as it is
clicked, hence the problem).

- Removed duplicate code; the movement is now managed by
QGraphicsItemGroup::mouseMoveEvent. This prevents use of the transform()
method to get the stroke's transformation matrix; so sceneTransform() is
used instead when copying a strokes group.

- Also fixed an oversight in UBBoardView: Media items couldn't be moved
directly anymore.