fonts: fontawesome integration: Include parts of fontawesome free "for the web" 6.6.0, which will be used for icons in frontends to replace fontello. Fontello is not maintained anymore (last commit 2 years ago), and has licensing issues (some icons were advertise as free while they were not). It will be removed from this repository, and fontawesome is used instead. Only the parts actually used in Libervia frontends are included, to save space.