Filter entities in the UI (part 0): Make CustomContent more useful

[abey79]

Related

• Part of Project: filtering entities in the UI (first iteration) #8586
• Part of a series of PR:
  • Filter entities in the UI (part 0): Make CustomContent more useful #8645
  • Filter entities in the UI (part 1): Introduce a filter widget #8652
  • Filter entities in the UI (part 2): Introduce entity filtering in the time panel #8654
  • Filter entities in the UI (part 3): Move action to a menu in the blueprint panel and keep default blueprint when using heuristics #8672
  • Filter entities in the UI (part 4): Add entity filtering in the blueprint tree #8706
  • Filter entities in the UI (part 5): Add snapshot tests for the blueprint tree #8728
  • Filter entities in the UI (part 6): Refactor re_blueprint_tree and add more tests #8795
  • Filter entities in the UI (part 7): Refactor and optimize re_time_panel and add more tests #8863
  • Filter entities in the UI (part 8): Make the query semantics more powerful #8886

What

This PR makes CustomContent for list items more useful, because I'll needed for the UI of the filter widget. I also cleans up a few things and updates the related snapshot test.

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Result	Commit	Link	Manifest
✅	c3ee2e0	https://rerun.io/viewer/pr/8645	+nightly +main

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[Wumpf]

neat!
makes me so happy that this is right away handled by a screenshot test

[Wumpf]

having a bit of a deja vu - isn't this button handling code already somewhere else? (can't tell without digging if that's actually the case and whether that's even a concern)

[abey79]

Yes, I already have some kind of abstraction for button thingies (could be menu, could be action). Look for the ItemButton trait. I'm using that in PropertyContent, so I'm piggy backing on it.