Stacking font-awesome icons

Font awesome from version 4 onwards provides an icon stacking syntax. This allows to get a way larger icon diversity than the simple icons. This would allow font awesome to match a lot of 'use specific' icons in Silk such as `add file` or `remove attachment`. Moreover, this would allow to build clean icon variations easily (no need to set side by side two icons anymore, they can be combined in a more meaningful way).

References:

https://fontawesome.com/docs/web/style/stack

http://bluegalaxy.info/codewalk/2018/04/04/html-css-stack-font-awesome-icons/