Polish image API (#3338)

* Imoprove docs for callback shapes

* Improve docs for loader traits

* Use snake_case for feature `all_loaders`

* Make loaders publix

* Slightly better error message on image load failure

* Improve image loading error messages

* Use `bytes://` schema for included bytes loader

* Try user loaders first

* Move `image_loading_spinners` to `Visuals`

* Unify and simplify code

* Make the main text of `Button` optional

This largely makes ImageButton obsolete

* Fix docstrings

* Better docs

* typos

* Use the more explicit `egui_extras::install_image_loaders`

* Simplify `Image::paint_at` function

crates/egui_extras/src/loaders/ehttp_loader.rs

@@ -72,7 +72,7 @@ impl BytesLoader for EhttpLoader {
                     bytes: Bytes::Shared(file.bytes),
                     mime: file.mime,
                 }),
-                Poll::Ready(Err(err)) => Err(LoadError::Custom(err)),
+                Poll::Ready(Err(err)) => Err(LoadError::Loading(err)),
                 Poll::Pending => Ok(BytesPoll::Pending { size: None }),
             }
         } else {