FEATRUE: Update operator dock properly when active model changes

For now operator dock(OP) would not be updated when switching active model.

It causes a downstream bug that if we add an image to a discrete model then add another image to a polygon model, model tree would not update properly since OP only update discrete model's descriptive phrase.(You need to switch to view by topology and back to view by list to see the udpate)

New logic is added so that OP would update model tree properly and clear misleading operators.(In the past with an active polygon model, we are still showing operators for discrete model)

close #139 (closed)