Add intensity to the feature widget

[AnniekStok]

Needs funkelab/funtracks#274

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.52941% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.96%. Comparing base (fee435a) to head (9b42eda).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...motile_tracker/application_menus/feature_widget.py	98.52%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #482      +/-   ##
==========================================
+ Coverage   88.87%   88.96%   +0.09%     
==========================================
  Files          59       59              
  Lines        5815     5883      +68     
==========================================
+ Hits         5168     5234      +66     
- Misses        647      649       +2     

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[cmalinmayor]

I think we can simplify this a bit - I'm not sure we need to worry about users incrementally updating the image layers associated with the Tracks. And, we can treat all the channels as one toggle, since computing the mean intensity of another channel is relatively cheap once we are calling regionprops regardless.
Proposed workflow:

• Setter that lets users assign image layers to the tracks. Usually only called once. Sets up the intensity feature with the proper number of channels. Lazy arrays are stored in the RegionpropsAnnotator, so we don't need to worry about renaming the layer
• Toggle button for the "image intensity" feature as whole, for all channels. Enables/disables the intensity feature.
• To change the intensity image(s), re-call the setter, reset the intensity feature, re-compute everything