Implement "Standard Button" using Control Themes

[erri120]

This was mostly an experiment to see how well the new Control Themes work in Avalonia.

Part of #735

[codecov]

Codecov Report

Merging #734 (94fe16f) into main (6df4e16) will increase coverage by 64.07%.
Report is 19 commits behind head on main.
The diff coverage is 3.57%.

@@            Coverage Diff            @@
##           main     #734       +/-   ##
=========================================
+ Coverage      0   64.07%   +64.07%     
=========================================
  Files         0      581      +581     
  Lines         0    17796    +17796     
  Branches      0     1247     +1247     
=========================================
+ Hits          0    11402    +11402     
- Misses        0     6073     +6073     
- Partials      0      321      +321     

Flag	Coverage Δ
Linux	63.36% <3.57%> (?)
Windows	63.16% <3.57%> (?)
clean_environment_tests	64.05% <3.57%> (?)

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

Files	Coverage Δ
src/NexusMods.App.UI/Palatte/Nexus.xaml	99.20% <100.00%> (ø)
src/NexusMods.App.UI/Icons/IconTypeConverter.cs	0.00% <0.00%> (ø)
...exusMods.App.UI/Controls/Buttons/StandardButton.cs	0.00% <0.00%> (ø)
...c/NexusMods.App.UI/Converters/TextCaseConverter.cs	0.00% <0.00%> (ø)

... and 577 files with indirect coverage changes