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_demo_app/src/wrap_app.rs

@@ -165,7 +165,7 @@ pub struct WrapApp {
 
 impl WrapApp {
     pub fn new(_cc: &eframe::CreationContext<'_>) -> Self {
-        egui_extras::loaders::install(&_cc.egui_ctx);
+        egui_extras::install_image_loaders(&_cc.egui_ctx);
 
         #[allow(unused_mut)]
         let mut slf = Self {