Add an isolation mode to the Path tool

There should be a checkbox added to the Path tool control bar. When active, it fades out all the non-selected layers. Editing is constrained to the selected layers only, and selection can't target any other layers unless the Ctrl modifier key is pressed. This ensures the user doesn't have to fight to select the intended paths without accidentally selecting other layers.

Fading out the non-selected layers might be challenging to figure out a way of implementing.