Improve clippy, and add more docs (#3306)

* Silence a few clippy warnings

* Use named threads

* Remove some deprecated functions

* Document Context and Ui fully

* Use `parking_lot::Mutex` in `eframe`

* Expand clippy.toml files

* build fix

crates/epaint/src/bezier.rs

@@ -1,4 +1,6 @@
 #![allow(clippy::many_single_char_names)]
+#![allow(clippy::wrong_self_convention)] // False positives
+
 use std::ops::Range;
 
 use crate::{shape::Shape, Color32, PathShape, Stroke};