fix: Fix emmision when signal disconnect itself

[Czaki]

I napari overlays we have a signal that need to disconnect itself (as it need to be executed only when event is first emited)

This PR is changing to not iterate over self._slots but list(self._slots) that means that disconnection during iteration is not skipping event just after disconnected one.

https://napari.zulipchat.com/#narrow/channel/212875-general/topic/Fix.20first.20.60visible.60.20event.20execution.20of.20overlays/with/588565203
closes napari/napari#8911
closes napari/napari#8910

The alternative might be to add an argument emmit_once to connect method.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.75%. Comparing base (1e90319) to head (409783b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #413   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files          23       23           
  Lines        2462     2462           
=======================================
  Hits         2456     2456           
  Misses          6        6           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed-hq]

Merging this PR will not alter performance

✅ 67 untouched benchmarks

---

_{Comparing Czaki:fix_disconnect (409783b) with main (1e90319)}