fix: ensure hide_sliders runs on the main thread

[tlambert03]

after searching for a while, I don't have a satisfying explanation for why this is happening... but following a dims_changed signal from a custom data wrapper, I'm getting

WARNING: Cannot make QOpenGLContext current in a different thread
zsh: abort      python examples/pymmcore_plus_example.py tensorstore

and this fixes it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.29%. Comparing base (d60f273) to head (f998b14).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   86.09%   86.29%   +0.19%     
==========================================
  Files          46       46              
  Lines        5208     5209       +1     
==========================================
+ Hits         4484     4495      +11     
+ Misses        724      714      -10     

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