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

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (4f088ab) 99.54% compared to head (6f9aece) 99.16%.

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     

Files	Coverage Δ
src/app_model/backends/qt/_qaction.py	100.00% <100.00%> (ø)
src/app_model/backends/qt/_qmenu.py	96.29% <100.00%> (ø)
src/app_model/backends/qt/_util.py	100.00% <100.00%> (ø)
src/app_model/_app.py	96.10% <70.00%> (-3.90%)	⬇️
src/app_model/types/_icon.py	90.00% <71.42%> (-10.00%)	⬇️

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