Fix target for click on progress bar

The progress bar shows how many slides have been passed in total.
However, when clicking on the progress bar, the target slide is
computed among the subset of /horizontal/ slides.  Thus, when the new
slide is displayed, the progress bar has usually changed to a point
that is unrelated to the clicked one, which I find surprising.

With this change, the target slide is computed from the number of
total slides.  Thus, after a click on the progress bar, the resulting
progress is close to the clicked point, which seems more natural to
me.