Gestures overhaul, part 2

See !605 (closed) and !821 (merged)

This change limits scrolling speed in overview and app grid to one page per 250ms. This should make it reasonable for mice and similar devices. It currently also does it for touchpads, part 3 will replace that with stick-to-finger scrolling.

The idea is to not allow scrolling while there's already scrolling animation going, but that fails when animations are disabled. Hence use a timeout with the same duration as that animation, so it does the same thing normally, but doesn't change behavior with slowdown/no animations.

Also reduce app grid page switch animation time to match overview.

Currently includes !821 (merged)

Fixed mouse parts of #1338 (closed)