Added Alt click modifier for selecting upstream nodes in GraphGadget.

Alt+Shift click adds all upstream nodes to the selection. Alt+Ctrl click removes all upstream nodes from the selection. Alt click alone remains the domain of the ViewportGadget camera controller, but the ViewportGadget now passes through Alt+Something presses, not using them for camera motion any more.

Also added GraphGadget::upstreamNodeGadgets() public method which is likely to be handy in the future, and was necessary for implementing this.

Fixes #437.