Navigation, zoom, and GNOME 51
- Adds navigation buttons for navigating back and forwards.
- Adds zoom controls: keyboard shortcuts, Ctrl+scroll, and a zoom row in the main menu.
- Adds a more helpful start screen, as well as loading and error screens.
- Remembers fava's settings. Specifically, by running fava on the same port if possible,
favagtk helps fava retrieve its last settings from localstorage.
- Updates fava to 1.30.16
- Updates the base image to GNOME 51
