Serde feature: Add serde derives to input related structs (#4100)

We plan to store input data for creating automated tests, hence the need
for more serde derives on input related structs.

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Co-authored-by: Georg Weisert <georg.weisert@freshx.de>

crates/emath/src/history.rs

@@ -16,6 +16,7 @@ use std::collections::VecDeque;
 /// or for smoothed velocity (e.g. mouse pointer speed).
 /// All times are in seconds.
 #[derive(Clone, Debug)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct History<T> {
     /// In elements, i.e. of `values.len()`.
     /// The length is initially zero, but once past `min_len` will not shrink below it.