clock: Monitor the wall clock w/o libgnome-desktop-3

This removes the minor dependency on libgnome-desktop-3, as well as being a slightly more reliable wall clock: rather than using g_timeout_add, it uses timerfds, as recommended by systemd (see org.freedesktop.timedate1(8)). In other words, it can easily notice when the system clock is changed and update itself accordingly.

(This is a version of !24 (closed) not using the separate fork.)