add _top_level to UserUpdateSegmentation and UserDeleteNodes to support copying of labels

[AnniekStok]

Some copy label actions are both painting and erasing at the same time, on different regions. They replace the clicked node entirely, but only write on some of the original pixels. Since the UserUpdateSegmentation action can only paint with exactly one value, this becomes two actions. In order to undo both actions in one go, we need the _top_level argument, so we can stack them together.

For motile tracker PR funkelab/motile_tracker#477

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (be685db) to head (3ea1d2f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #271   +/-   ##
=======================================
  Coverage   93.92%   93.92%           
=======================================
  Files          59       59           
  Lines        3323     3326    +3     
=======================================
+ Hits         3121     3124    +3     
  Misses        202      202           

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