Somebody standing at a display who wants to hold a slide, or go back to one that
has just passed, had to wait for the whole rotation to come round again.
Left and right arrows step back and forward; PageUp and PageDown do the same, so
a presenter remote works without being told about it. The listener is on window
rather than an element because a display has nothing focused.
Stepping restarts the rotation timer instead of leaving it running. Advancing by
hand and then having it move again a second later, because the existing timer
was nearly up, reads as the display ignoring the keypress.
Ignored entirely when there is less than one slide to move to.
a18335b61c