Add settings for custom UI fonts

[marcoms]

Added basic support for changing the sans-serif + serif fonts used in the UI via Settings.toml. It currently re-uses the reader.font-path setting for backwards compatibility, but perhaps this could be a separate toplevel setting as well. This can also be extended to the other variants like monospace etc, but thought it was worth creating a POC for now.

Example config + result:

font-family-sans = "Inter"
font-family-serif = "Copernicus"

(Let me know if this is out of scope for the project)

Fixes #389