Fixed Backdrop cut+paste bug.

More specifically, backdrops were selecting their children (framed nodes) whenever they themselves were selected, regardless of the source of that selection event. When a backdrop was being pasted, it was being pasted over the top of the original, getting selected, selecting all it's children (including the ones from the original, because they were contained in the bounds) and then getting moved to the new location, along with the bogus selected nodes.

This fix makes sure that the child nodes are only selected if the node is selected immediately following a click, and not at any other time. I'm not particularly fond of the mechanism, but it's definitely better than no fix. Alternatives would be making the GraphGadget more aware of Backdrops, or duplicating the GraphGadget selection logic in the Backdrop node - if anyone has strong opinions either way I can try one of those appropaches instead.

Fixes #539.