Rust 1.60.0 + refactor cargo features (#1467)

* Update to rust 1.60.0
* Rename the feature `convert_bytemuck` to `bytemuck`
* Rename the feature `serialize` to `serde`.
* Make use of the "weak dependency" cargo feature
* Set rust-version = "1.60" for all crates
* egui_glow: clipboard, links, persistence & winit are now opt-in features

egui_web/src/screen_reader.rs

@@ -4,6 +4,7 @@ pub struct ScreenReader {
 }
 
 #[cfg(not(feature = "screen_reader"))]
+#[allow(clippy::derivable_impls)] // False positive
 impl Default for ScreenReader {
     fn default() -> Self {
         Self {}