Refactor callback handling

[dstansby]

No description provided.

[codecov-commenter]

Codecov Report

Merging #33 (043ffb3) into main (234227d) will increase coverage by 1.93%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   90.65%   92.59%   +1.93%     
==========================================
  Files           7        7              
  Lines         107      108       +1     
==========================================
+ Hits           97      100       +3     
+ Misses         10        8       -2     

Impacted Files	Coverage Δ
src/napari_matplotlib/base.py	100.00% <100.00%> (ø)
src/napari_matplotlib/histogram.py	100.00% <100.00%> (+9.67%)	⬆️
src/napari_matplotlib/scatter.py	64.70% <100.00%> (-16.07%)	⬇️

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