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/egui_demo_app/src/backend_panel.rs

@@ -451,10 +451,13 @@ impl EguiWindows {
 
 #[cfg(not(target_arch = "wasm32"))]
 fn call_after_delay(delay: std::time::Duration, f: impl FnOnce() + Send + 'static) {
-    std::thread::spawn(move || {
-        std::thread::sleep(delay);
-        f();
-    });
+    std::thread::Builder::new()
+        .name("call_after_delay".to_owned())
+        .spawn(move || {
+            std::thread::sleep(delay);
+            f();
+        })
+        .unwrap();
 }
 
 #[cfg(target_arch = "wasm32")]