paginator: Support discrete scrolling

Support scrolling on devices like mice. Switch a page when a scroll event arrives and add a delay to prevent too fast scrolling.

Use animation duration as a delay, but don't let it go below 250ms, mainly to ensure it still works with animations disabled.

Switch trackpoint to use discrete scrolling too, as swiping may be awkward.

Fixes https://source.puri.sm/Librem5/libhandy/issues/155

Signed-off-by: Alexander Mikhaylenko alexm@gnome.org