Easing functions (#4630)

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).

crates/emath/src/lib.rs

@@ -26,6 +26,7 @@ use std::ops::{Add, Div, Mul, RangeInclusive, Sub};
 // ----------------------------------------------------------------------------
 
 pub mod align;
+pub mod easing;
 mod history;
 mod numeric;
 mod ordered_float;