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/svg_loader.rs

@@ -42,7 +42,7 @@ impl ImageLoader for SvgLoader {
         if let Some(entry) = cache.get(&(uri.clone(), size_hint)).cloned() {
             match entry {
                 Ok(image) => Ok(ImagePoll::Ready { image }),
-                Err(err) => Err(LoadError::Custom(err)),
+                Err(err) => Err(LoadError::Loading(err)),
             }
         } else {
             match ctx.try_load_bytes(&uri) {
@@ -60,7 +60,7 @@ impl ImageLoader for SvgLoader {
                     cache.insert((uri, size_hint), result.clone());
                     match result {
                         Ok(image) => Ok(ImagePoll::Ready { image }),
-                        Err(err) => Err(LoadError::Custom(err)),
+                        Err(err) => Err(LoadError::Loading(err)),
                     }
                 }
                 Ok(BytesPoll::Pending { size }) => Ok(ImagePoll::Pending { size }),