Need to use `Enum` in a better way

[qin-yu]

I needed to fix "MutexWS" name/val correspondence because in aggl. task "gasp", "mutex_ws", "multicut" are matched as strings, while in another module for GUI "mutexws" was used.

We better make a Enum instead of matching strings in many cases in PlantSeg. But some refactored Enums in PlantSeg are really hard to use. Need to refactor and make a generic class with to_choice(). The challenge has been that in Napari GUI the choice should have a human readable string as the name and a constant code as its value, this is the reverse of Enum.

Or we find an alternative.