This adds most of the "standard" easing functions from
https://easings.net/ to `emath::easing`, and adds helpers in `egui` for
using them.
In particular there is now `ctx.animate_bool_with_easing` and
`ctx.animate_bool_responsive`, that uses a cubic easing function.
All animations in egui now uses cubic ease-out, for a more responsive
feeling (fast at the start, slower towards the end).
29b12e1760