Windows High Contrast Mode support

[ffoodd]

If that's something to do, let's do it :)

Some refrences:

• https://adrianroselli.com/2012/08/css-background-images-high-contrast-mode.html
• https://www.scottohara.me/blog/2019/02/12/high-contrast-aria-and-images.html
• https://developer.paciellogroup.com/blog/2016/12/windows-high-contrast-mode-the-limited-utility-of-ms-high-contrast/
• http://gwhitworth.com/blog/2019/08/high-contrast-text-backplate/
• https://blogs.windows.com/msedgedev/2020/09/17/styling-for-windows-high-contrast-with-new-standards-for-forced-colors/

Other references from #209-Contrast enforcement widget:

• https://github.com/Orange-OpenSource/Orange-Boosted-Bootstrap/issues/209#issue-490206572
• https://github.com/Orange-OpenSource/Orange-Boosted-Bootstrap/issues/209#issuecomment-529976633