the lag / skipping of the clock was not caused by faulty timer code
on fluxbox's side but by the behavior and inner workings of time().
since this is fixed now (913244789f) we can now rollback ec7fe513c8
and detect strftime-formats which need intervals of seconds or minutes.
minor: the small change to FbTk::Timer::setTimeout() reduces one
start() / stop() cycle for a running timer.
