Use Liberation Serif instead of DejaVu Serif as a fallback for Georgia

DejaVu Serif has different metrics (wider) than Georgia. This results in a line break in the menu on Linux for me. I think `Liberation Serif` is a better fallback.

(Also note that serif defaults to `DejaVu Serif` on most Linux systems so there's no change at all for users who haver neither Georgia nor Liberation Serif)