Add button.clicked alias for button.changed

[tlambert03]

closes #266

[codecov]

Codecov Report

Merging #271 (a0566ab) into master (481f665) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
- Coverage   90.28%   90.26%   -0.03%     
==========================================
  Files          28       28              
  Lines        3182     3185       +3     
==========================================
+ Hits         2873     2875       +2     
- Misses        309      310       +1     

Impacted Files	Coverage Δ
magicgui/widgets/_bases/button_widget.py	96.42% <66.66%> (-3.58%)	⬇️

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