There's no point stacking the various style changes successively -- all
of them will be applied on the next defer. So, just ensure there is a
single defer before announcing the page change.
Also, we can make the slide faster if there's been another slide very
recently -- the user is obviously paging through the book quickly; let's
not get in their way.
I've always hated it. Now should only appear if the next page is
actually taking a while to set (because it has a slight transition on
the opacity, so near-zero setPage times shouldn't ever let the opacity
reach 1). Need to test this on slower devices.