Split widget bases into files

[tlambert03]

starting to want the _bases.py split into different files

[codecov]

Codecov Report

Merging #114 (c2e93af) into master (e1f8895) will increase coverage by 0.24%.
The diff coverage is 90.45%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   88.07%   88.32%   +0.24%     
==========================================
  Files          15       24       +9     
  Lines        2315     2364      +49     
==========================================
+ Hits         2039     2088      +49     
  Misses        276      276              

Impacted Files	Coverage Δ
magicgui/widgets/_bases/categorical_widget.py	74.64% <74.64%> (ø)
magicgui/widgets/_bases/ranged_widget.py	82.02% <82.02%> (ø)
magicgui/widgets/_bases/widget.py	90.57% <90.57%> (ø)
magicgui/widgets/_bases/mixins.py	90.90% <90.90%> (ø)
magicgui/widgets/_bases/container_widget.py	95.75% <95.75%> (ø)
magicgui/widgets/_bases/create_widget.py	97.14% <97.14%> (ø)
magicgui/widgets/_bases/value_widget.py	97.77% <97.77%> (ø)
magicgui/widgets/_bases/__init__.py	100.00% <100.00%> (ø)
magicgui/widgets/_bases/button_widget.py	100.00% <100.00%> (ø)
magicgui/widgets/_bases/slider_widget.py	100.00% <100.00%> (ø)
... and 10 more

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