Make placeholders show during fast swipe/flick

# Problem
The placeholders do not show during a flick, showing the background of the page.  It looks icky.

# Solution
Flipping the sign when the translations go into positions, so that the placeholder calculation works correctly.

Also, sidebar fix: I halved the swipe animation time so that the ease out at the end didn't feel like eternity.

CR: @timmccall-wf @lancefisher-wf @robbecker-wf @patkujawa-wf @georgelesica-wf @todbachman-wf 

I think ideally, this would stay private if nothing else uses it.