Add shortcuts dialog to help menu

[jeylau]

[codecov-commenter]

Codecov Report

Merging #101 (b857b1d) into main (44abe9c) will decrease coverage by 0.38%. The diff coverage is 40.00%.

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@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
- Coverage   82.78%   82.41%   -0.38%     
==========================================
  Files          11       11              
  Lines        1719     1734      +15     
==========================================
+ Hits         1423     1429       +6     
- Misses        296      305       +9     

Files Changed	Coverage Δ
src/napari_deeplabcut/_widgets.py	72.90% <40.00%> (-0.59%)	:arrow_down:

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[MMathisLab]

nice!