manifest: Add gnome-shell-extensions
manifest: Add gnome-shell-extensions

This syncs us up with the jhbuild moduleset.

vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,chergert/gnome-continuous,GNOME/gnome-ostree,GNOME/gnome-ostree,vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree,djdeath/gnome-ostree,GNOME/gnome-ostree