feat: improve cmap combobox generalization and add right-click-to remove

[tlambert03]

this PR:

1. makes QColormapComboBox compatible with QlementineStyle (without changing how it looks normally), by using more generic QStyle APIs
2. adds a right-click-to-remove from the options... so you can easily add/remove colormaps from the options

[codecov]

Codecov Report

❌ Patch coverage is 52.17391% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.81%. Comparing base (60cc8cf) to head (02986b1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/superqt/cmap/_cmap_combo.py	38.88%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #333      +/-   ##
==========================================
- Coverage   85.24%   84.81%   -0.44%     
==========================================
  Files          49       49              
  Lines        3871     3904      +33     
==========================================
+ Hits         3300     3311      +11     
- Misses        571      593      +22     

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