Fixed NodeGraph framing bug.

It was framing every time a node was moved. This was happening because I moved the dropSignal connection from self to __gadgetWidget in the recent rejig "for consistency". But because the GadgetWidget generates drags for moving nodes, but doesn't handle drops, we were ending up with drops which we didn't want.
Merge pull request #612 from johnhaddon/nodeGraphFramingBug

Fixed NodeGraph framing bug.