Add SliceSelector

[dstansby]

Fixes https://github.com/dstansby/napari-matplotlib/issues/11

Still to do:

• Add a test
• Add hooks to the spin box widgets to update the plots
• Add hook to update the viewer z value when the z spin box is updated

[codecov-commenter]

Codecov Report

Merging #29 (23d7531) into main (043ffb3) will decrease coverage by 18.93%.
The diff coverage is 38.98%.

@@             Coverage Diff             @@
##             main      #29       +/-   ##
===========================================
- Coverage   92.59%   73.65%   -18.94%     
===========================================
  Files           7        8        +1     
  Lines         108      167       +59     
===========================================
+ Hits          100      123       +23     
- Misses          8       44       +36     

Impacted Files	Coverage Δ
src/napari_matplotlib/base.py	100.00% <ø> (ø)
src/napari_matplotlib/slice.py	37.93% <37.93%> (ø)
src/napari_matplotlib/__init__.py	71.42% <100.00%> (+4.76%)	⬆️

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