Refactor polygon edge operators and related UI

Move edge creation by widgt feature out of edit-edge operator into create-edge, so now create-edge has a new option to create edge by widget. Also, added cusomized UI components for split-edge operator. All three operators (create-edge, edit-edge, split-edge) now use customized view, qtPolygonEdgeOperationView, which manages ui panels based on which operator is being inovoked.