Closest value/time on a curve

[LeXXik]

It is often needed to get a closest point on a curve to some world point. This is a bit cumbersome to do today, so this is a feature request. We need a method that takes a value and returns closest value or time.

There is a private Curve.closest(time) method, which doesn't seem to be used anywhere. Anyhow, it just returns a closest key, which is not that useful, as I know the keys already. It also takes time, which is not known at first. Maybe a candidate to deprecate.