Emit signal only if value of caterogical widget changed

[Czaki]

This is bugfix PR for not emmiting signal that value is changed if nothing changed in selection.

[codecov]

Codecov Report

Merging #422 (4c65e59) into main (d40bb0e) will increase coverage by 0.12%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main     #422      +/-   ##
==========================================
+ Coverage   88.81%   88.94%   +0.12%     
==========================================
  Files          30       30              
  Lines        3917     3926       +9     
==========================================
+ Hits         3479     3492      +13     
+ Misses        438      434       -4     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.88% <92.85%> (+0.62%)	⬆️

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[tlambert03]

This looks good to me. Sorry for the relatively convoluted stuff going on here. I can merge this and cut 0.5.1 if you're happy with it

[Czaki]

All my test works with this change.