#81 Move to closest position on illegal node moves

[Peter Becker]

The current behaviour of avoiding illegal states of the
diagram makes it a bit jumpy and unpredictable.
Whenever a node movement is illegal, the system should
not just reject it, but move to the closest legal
position instead. This way it would be easier to get to
the boundary situation and the movement would be less
jumpy.

To understand "jumpy" do this: drag a node, move the
mouse too high, then off to the side while staying too
high, then straight downwards until the position gets
legal.