Add support for better font fallback

[LaurenzV]

There have been numerous issues (e.g. #43 #44 #63) where people have had issues with text not showing up. Those are all very likely because the exact font isn't installed on the system (or isn't found) and font fallback fails to find another appropriate one.

Since we have logging support now, we should at least get warnings if that happens, but in the end we still need better fallback support. However, since text conversion is done by usvg, there isn't much we can do from our side for now...