Added value drag/drop to SceneInspector.
Added value drag/drop to SceneInspector.

Fixes #830.

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