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.
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