[a11y] Feature Accesibility High Contrast 

- Add Optional High Contrast theme, disabled by default.
- Accesibility Universal Access oriented feature.
- Update settings.
- Update translations.
- Add checkbox on Preferences to enable/disable.
- Add QSS Black/White theme.
- Make sure it does not break UI.
- This feature can be extended on future *(one by pull request)*

Real Screenshots of how it looks like:
https://plus.google.com/118074123761587415902/posts/5GwRkmUSnjB

brancho:  feature/a11y

:tangerine: