Bugfix: fix broken "show selected" in the Labels layer (because of caching)

[ksofiyuk]

Description

The labels caching from #5732 broke the "show selected" feature because of caching. This PR fixes it by invalidating the cache when "show selected" is activated.

I also added a test that catches this bug.

Type of change

• Bug-fix (non-breaking change which fixes an issue)

How has this been tested?

• all tests pass with my change

Final checklist:

• My PR is the minimum possible work for the desired functionality
• I have added tests that prove my fix is effective or that my feature works

[codecov]

Codecov Report

Merging #5841 (dec9c27) into main (0dfb37b) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5841      +/-   ##
==========================================
- Coverage   89.96%   89.94%   -0.03%     
==========================================
  Files         615      615              
  Lines       52638    52654      +16     
==========================================
+ Hits        47358    47359       +1     
- Misses       5280     5295      +15     

Impacted Files	Coverage Δ
napari/layers/labels/_tests/test_labels.py	99.52% <100.00%> (+<0.01%)	⬆️
napari/layers/labels/labels.py	95.87% <100.00%> (+0.01%)	⬆️

... and 6 files with indirect coverage changes

[brisvag]

Nice, thanks for adding the test as well 💪

[jni]

Thanks @ksofiyuk 🙏