Add Radiobuttons to select filter for FileEdit

[fdsteffen]

Addresses #36 to set a file filter for the FileEdit widget when reloading a classifier.

[jluethi]

Thanks a lot @fdsteffen ! Looks good to me and also works in my tests.
I added a bit of extra documentation so that we remember why this picker is there. Neat that it only needs changes to the loader widget, so the main widgets aren't made more complex.

Will merge once tests pass again :)

Edit: I made tests fail with the code coverage reporter. Actually good reminder to add some basic tests to that loader class, will look into this :)

[codecov-commenter]

Codecov Report

Merging #37 (63565a0) into main (710a801) will decrease coverage by 0.31%.
The diff coverage is 16.66%.

❗ Current head 63565a0 differs from pull request most recent head 084721e. Consider uploading reports for the commit 084721e to get more accurate results

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
- Coverage   82.37%   82.07%   -0.31%     
==========================================
  Files          11       11              
  Lines         749      753       +4     
==========================================
+ Hits          617      618       +1     
- Misses        132      135       +3     

Files Changed	Coverage Δ
src/napari_feature_classifier/classifier_widget.py	77.88% <16.66%> (-1.04%)	⬇️

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[jluethi]

Ok, with those small tests added, I think that should be good enough coverage for now :) Can't figure out how to update the coverage report here, but now we have loading at least covered with some basic tests.

Merging now, thanks again @fdsteffen