Previously, we used a timer with a timeout of 0 for redrawing the wibox. I had
the visual impression that the wibox was black for a moment. With strace I was
able to measure a latency of 10ms until the wibox was finally redrawn.
This now uses the "refresh" signal. With this, we get our latency down to
something like 0.15ms which sounds a lot better. :)
Signed-off-by: Uli Schlachter <psychon@znc.in>