Delete button when removing from qt RadioGroup

[tlambert03]

fixes #197

[codecov]

Codecov Report

Merging #198 (27e22bd) into master (c50a7e2) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   90.07%   90.20%   +0.13%     
==========================================
  Files          29       29              
  Lines        2981     2982       +1     
==========================================
+ Hits         2685     2690       +5     
+ Misses        296      292       -4     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	87.11% <100.00%> (+0.23%)	⬆️
magicgui/application.py	83.14% <0.00%> (+2.24%)	⬆️
magicgui/backends/_qtpy/application.py	100.00% <0.00%> (+2.77%)	⬆️

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