Bump our GLib dependency to 2.60 so we can use `gdbus-codegen
--interface-info-{body,header}` to generate interface definitions
dynamically rather than hand-coding them.
We actually need to depend on 2.60.1 so we get
https://gitlab.gnome.org/GNOME/glib/merge_requests/721.
Signed-off-by: Philip Withnall <withnall@endlessm.com>