Backport page switcher

There is a much nicer mobile page switcher in Epiphany upstream, which would be cool to have on the devkit image:

This is the main MR, though I think there are some followup ones: https://gitlab.gnome.org/GNOME/epiphany/merge_requests/275

There is also a nice counter on the tab button now: https://gitlab.gnome.org/GNOME/epiphany/merge_requests/318