Invert selection, and hide everything not selected

[dustenhubbard]

Problem

Proofreading a large auto-seg ROI is sluggish because all traces stay live; flipping, deleting, and merging lag. Users want to focus on one object (or a few) and push the rest out of the way.

Proposed

(1) An invert selection action that flips selected/unselected. (2) A hide unselected / show-only-selected toggle to isolate what you're working on. Pairs with the existing Hide (H) behavior.

Notes

Originally floated by Michael (~a year ago, "hide other windows"), re-requested by Weiling for auto-seg proofreading. Must respect existing undo (Ctrl+Z) semantics for object/trace-list actions.

Type:

Enhancement.

[quasiTriestino]

Still learning the ropes here. Not sure how to merge after editing through on my machine, so I didn't incorporate this PR cleanly; though, these changes and now incorporated into main as of 381df15.