Fix generated graphs being off the canvas.

 - When using the generate-graph tool with an empty canvas,
   most of the generators produce graphs with some nodes
   with negative X or Y coordinates; these lie outside of
   the canvas and are not shown in the UI. This is confusing.

Introduce a (template) method that checks if a given iterable
collection of nodes lies outside of the canvas, and if so
translates (slides) the entire collection by enough so that
it does lie inside the canvas.

Use this method in all the graph types that have an identifiable
NodePtr collection -- the random graphs are not fixed yet.