Add multiple anchor points

[thorstenwagner]

This PR adds the option to add multiple red circles ("anchor points") to the umap by holding shift.

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (6e1c552) 36.53% compared to head (b12d49e) 35.18%.

Files	Patch %	Lines
src/napari_tomotwin/load_umap.py	0.00%	11 Missing ⚠️
src/napari_tomotwin/make_targets.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   36.53%   35.18%   -1.36%     
==========================================
  Files           5        5              
  Lines         208      216       +8     
==========================================
  Hits           76       76              
- Misses        132      140       +8     

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