gs-window-x11: use g_timeout_add_seconds for watch dog

The watch dog timer runs sort of arbitrarily every 30 seconds.
We don't need much precision on when it runs, it just needs to
run once in a while.

This commit changes the timer setup call from g_timeout_add
to g_timeout_add_seconds so the watch dog will wake up with
any other pending timeouts that happen to expire around the same
time, saving a little power.

http://bugzilla.gnome.org/show_bug.cgi?id=648145