Check OMERO color versus napari colormaps and use them if they exist

[psobolewskiPhD]

Closes: #77

Currently when an image is loaded the color from OMERO is used to create a new colormap every time.
In this PR I used a napari utility function that checks whether a colormap exists for the color and use that, else make a new colormap with the hex code of the color.
This works quite well for the images I've tested that use CYM, RGB. Nice QoL improvement.
I did run into an issue with White (which also happens for Black) where napari incorrectly doesn't realize it has that colormap (gray and gray_r) see napari/napari#7504. I've put in a workaround.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 33.01%. Comparing base (1d53966) to head (4f10b8b).

Files with missing lines	Patch %	Lines
src/napari_omero/plugins/loaders.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
- Coverage   33.13%   33.01%   -0.12%     
==========================================
  Files          13       13              
  Lines         845      848       +3     
==========================================
  Hits          280      280              
- Misses        565      568       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.