So far, multiple notes per slide are only supported if they are
attached to fragments (without fragments, only the first aside notes
element on a slide is displayed).  With this commit, the contents of
all notes on a slide are displayed (except for fragments with notes,
for which, as before, only each fragment's first/single note is
displayed).

When using the totalTime-based pacing calculation, a presenter may
inadvertently set totalTime and per-slide data-timing attributes in
such a way that the pacing time for some slides is impossibly low or
even negative.

Add a check to ensure that the pacing on a slide never falls below a
configurable minimum, defaulting to 0. Display an alert if the
pacing for any slide(s) falls below the threshold.