Misc code cleanup (#3398)

* eframe README: explain how to enable copy/paste

* Implement Debug for a couple of structs

* Code cleanup

* Better docs

* profile ron serialization

* CI: Allow "exclude from changelog" as the only label

crates/eframe/src/native/file_storage.rs

@@ -130,6 +130,7 @@ fn save_to_disk(file_path: &PathBuf, kv: &HashMap<String, String>) {
             let mut writer = std::io::BufWriter::new(file);
             let config = Default::default();
 
+            crate::profile_scope!("ron::serialize");
             if let Err(err) = ron::ser::to_writer_pretty(&mut writer, &kv, config)
                 .and_then(|_| writer.flush().map_err(|err| err.into()))
             {