Custom pseudo-elements support

[jarek-foksa]

Most custom CSS variables could be replaced with corresponding custom pseudo-elements.

This has to wait until Chrome implements support for custom pseudo-elements.

Documentation:

• https://github.com/w3c/webcomponents/issues/300
• https://github.com/w3c/webcomponents/blob/gh-pages/proposals/Custom-Pseudo-Elements.md
• https://tabatkins.github.io/specs/css-shadow-parts/

[jarek-foksa]

Done in Xel 0.9.