Update default favorite apps list
Many of the apps in the favorite apps list have fallen out of favor.
Replacing Rhythmbox with Music and Shotwell with Photos are obvious moves. Rhythmbox and Shotwell are not core apps, and that means we assume they are not installed by default. It doesn't really make sense to have non-default apps in the apps list.
Evolution is also not a core app, and that is not likely to change, so it should also be removed. Adding Geary might be more controversial. It is a strong candidate to become a core app, GNOME Mail, in the near future, but it is not there yet. So this could arguably be considered premature. But I figure a GSettings default is a cheap thing; we can always change it later if desired.