feat: support icon color

[tlambert03]

pulling the color-specific stuff off of #130

• does a better job of autodetecting the current theme mode in the qt backend (i.e. whether the current theme is "dark" or "light"
• allows the Application to explicit declare the active theme mode with Application.theme_mode = 'dark', which will then determine the color of icons generated thereafter (but see note below)
• adds color_dark/color_light fields to the Icon type to allow an icon to override the color that should be used in light/dark modes. (otherwise a dark gray / light gray default will be used in light / dark modes respectively)

note that svg-colored icons don't currently change if the parent style sheet changes. This is just a generally harder thing to solve (and one of the few benefits of font-icons). However, we could, as magicgui does, attach QPallete change events here and auto-change icon colors in a future PR.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.16%. Comparing base (4f088ab) to head (6f9aece).
⚠️ Report is 76 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app_model/types/_icon.py	71.42%	4 Missing ⚠️
src/app_model/_app.py	70.00%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (85.71%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
- Coverage   99.54%   99.16%   -0.38%     
==========================================
  Files          31       31              
  Lines        1761     1803      +42     
==========================================
+ Hits         1753     1788      +35     
- Misses          8       15       +7     

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