Fix target for click on progress bar

[lechten]

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.

[hakimel]

Nice improvement, thanks!