Supports homemade duotone icons

This MR supports duo tones icons.

The duo tone icons are not available for free. But we can provides our own from fontawesomemore https://ui.gitlab-pages.esrf.fr/fontawesomemore/ (see fam-bliss for example)

So "fa fam-flask" is the default icon, and "fad fam-flask" is the duo tone version. And there is a fall back if the duo tone version is not available.

This can be tuned with CSS vars

--fa-primary-color
--fa-primary-opacity
--fa-secondary-color
--fa-secondary-opacity

See https://fontawesome.com/docs/web/style/duotone