Fix off canvas

When using Graph Document -> Tools -> Generate Graph, on an empty canvas / whiteboard, the nodes of the graph (e.g. a star graph) are generated so that they fall off the whiteboard. Adjust the code so that generated nodes are all on the whiteboard (even though the circle representing the nodes may still be partly-cut-off).

While here, fix a re-layouting bug: generating a Random Tree or Random DAG will re-layout nodes that it didn't generate, which I think is a bug.