Added NodeGraph Ctrl+Click to select downstream nodes.
Added NodeGraph Ctrl+Click to select downstream nodes.

Fixes #941.

appleseedhq/gaffer,goddardl/gaffer,GafferHQ/gaffer,hradec/gaffer,johnhaddon/gaffer,boberfly/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,chippey/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,goddardl/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,hradec/gaffer,GafferHQ/gaffer,chippey/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,goddardl/gaffer,chippey/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,chippey/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,hradec/gaffer,ImageEngine/gaffer,boberfly/gaffer,GafferHQ/gaffer,hradec/gaffer,hradec/gaffer,boberfly/gaffer,ivanimanishi/gaffer,goddardl/gaffer