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/Cargo.toml

@@ -19,7 +19,7 @@ crate-type = ["cdylib", "rlib"]
 default = ["glow", "persistence"]
 
 http = ["ehttp", "image", "poll-promise", "egui_extras/image"]
-image_viewer = ["image", "egui_extras/all-loaders", "rfd"]
+image_viewer = ["image", "egui_extras/all_loaders", "rfd"]
 persistence = ["eframe/persistence", "egui/persistence", "serde"]
 web_screen_reader = ["eframe/web_screen_reader"]                  # experimental
 serde = ["dep:serde", "egui_demo_lib/serde", "egui/serde"]

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 {