Add close method

[tlambert03]

adds close method to base widget.

cc @hanjinliu (https://github.com/hanjinliu/magic-class/blob/41ebf386249d55a4665eefc28c651c34bd20a323/magicclass/basegui.py#L123)

[codecov]

Codecov Report

Merging #283 (7a1feef) into main (8d47eba) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
+ Coverage   89.96%   89.98%   +0.01%     
==========================================
  Files          29       29              
  Lines        3248     3254       +6     
==========================================
+ Hits         2922     2928       +6     
  Misses        326      326              

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	85.53% <100.00%> (+0.03%)	⬆️
magicgui/widgets/_bases/widget.py	87.93% <100.00%> (+0.14%)	⬆️
magicgui/widgets/_protocols.py	99.46% <100.00%> (+<0.01%)	⬆️

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