UI styling rework

[Al12rs]

Closes #735
Closes #721

This moves all our Styles to a dedicated Theme project, where Style classes are mostly being used.

Changes:

• Created dedicated Theme project
• Introduced new 3 layer Color system from Figma
• Introduced new Opacity layer system
• Implemented a ColorPalette for Avalonia Fluent Theme that provides good default foreground and background colors
• Consolidated Typography
  • Fixed a number of issues with Text styles, found cause of alignment issues (Avalonia Bug)
• Added many scattered icons to IconsStyles
• Added ADR for Styling approach choices
• Added Styling guidelines documentation
• Moved App.axaml from App.UI project to App project, to avoid Circular dependency App.UI -> Theme -> App.UI
• Added way to define BoxShadows using resources for colors instead of color literals
• Moved all Styling found in UI files to Theme project
• Moved all other existing styles to Theme project
• Reorganized folder structures for Styles in theme project for better organization
• Fixed and improved a whole bunch of styles (basically all of them). Some completely redone
• Removed old styles and old resources

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (9875f6c) 61.56% compared to head (b59f03d) 60.29%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #829      +/-   ##
==========================================
- Coverage   61.56%   60.29%   -1.28%     
==========================================
  Files         618      594      -24     
  Lines       19092    18130     -962     
  Branches     1396     1372      -24     
==========================================
- Hits        11754    10931     -823     
+ Misses       6992     6877     -115     
+ Partials      346      322      -24     

Flag	Coverage Δ
Linux	59.60% <97.18%> (-1.29%)	⬇️
Windows	59.43% <97.18%> (-1.34%)	⬇️
clean_environment_tests	60.27% <97.18%> (-1.28%)	⬇️
network_tests	?
networking_tests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...Mods.Games.FOMOD.UI/Resources/Language.Designer.cs	0.00% <ø> (ø)
...s/Download/SpineDownloadButtonDesignerViewModel.cs	47.36% <100.00%> (ø)
...e/Buttons/Download/SpineDownloadButtonViewModel.cs	100.00% <ø> (ø)
.../NexusMods.App.UI/Controls/Spine/SpineViewModel.cs	66.41% <100.00%> (ø)
src/NexusMods.App.UI/ReactiveUIExtensions.cs	58.82% <100.00%> (ø)
src/NexusMods.App.UI/Services.cs	98.40% <100.00%> (-0.02%)	⬇️
src/NexusMods.App/Services.cs	82.89% <100.00%> (+0.22%)	⬆️
src/NexusMods.App/Startup.cs	85.71% <ø> (ø)
....NexusFluentDark/Extensions/BoxShadowExtensions.cs	100.00% <100.00%> (ø)
...I/Window/AdvancedInstallerWindowDesignViewModel.cs	0.00% <0.00%> (ø)
... and 4 more

... and 2 files with indirect coverage changes

[Al12rs]

Fomod installer styling seems to be broken, I think I missed it. Will add that to the PR.

[erri120]

When opening the app for the first time

[Al12rs]

Fomod Stuff should be added now

[erri120]

The telemetry overlay needs to be fixed, and we should also look into getting auto-completion working.