Accessibility: Support system color appearance and theming in WebAssembly

Make sure to respect system setting in terms of light/dark mode and high contrast.

High Contrast

Discussion points:

• Do we want to go with HightContrast as an flag or enum? It is treated as enum of `{NoPreference, More, Less, Custom}` by W3: https://www.w3.org/TR/mediaqueries-5/#prefers-contrast
  It should probably go into StyleHints, next to ColorScheme
• Is this orthogonal to `ColorScheme` or part of it? 
  • should be orthogonal so we can express `Dark with high contrast` vs `Light with high contrast`
• On some systems like Mac high contrast setting is implemented purely by OS and is transparent to an application, we need to take care of respecting that
• Contrast preference should be read only property? We read it from the system, user should not be able to request it explicitly via Qt API?
• We need to come up with some API that would encompass wide range of underlying, a11y and high contrast implementation tends to vary a lot

Practicalities:

To work efficiently as a team we can divide and structure work into independent pieces which could be parallelized: 

1. Decide on type of `contrastPreference`, add its implementation to StyleHints.
2. Provide support of detecting contrast preferences for different platforms:
   1. Windows
   2. Gnome/KDE
   3. WebAssembly
   4. ...
3. Implement support for contrast preference in platform themes:
   1. Windows
   2. Gnome/KDE
   3. WebAssembly
   4. ...
4. Implement support for contrast preference in various styles:
   1. Fusion
   2. Adwaita
   3. Windows
   4. ...