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/lib.rs

@@ -34,6 +34,8 @@ pub use crate::sizing::Size;
 pub use crate::strip::*;
 pub use crate::table::*;
 
+pub use loaders::install_image_loaders;
+
 // ---------------------------------------------------------------------------
 
 mod profiling_scopes {