Add WidgetType::Image and Image::alt_text (#5534)

This adds `WidgetType::Image` and correctly sets it in the Image widget.
This allows us to query for images in kittest tests and tells accesskit
that a node is an image.
It also adds `Image::alt_text` to set a text that will be shown if the
image fails to load and will be read via screen readers. This also
allows us to query images by label in kittest.


* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_demo_app/src/apps/image_viewer.rs

@@ -14,6 +14,7 @@ pub struct ImageViewer {
     fit: ImageFit,
     maintain_aspect_ratio: bool,
     max_size: Vec2,
+    alt_text: String,
 }
 
 #[derive(Clone, Copy, PartialEq, Eq)]
@@ -44,6 +45,7 @@ impl Default for ImageViewer {
             fit: ImageFit::Fraction(Vec2::splat(1.0)),
             maintain_aspect_ratio: true,
             max_size: Vec2::splat(2048.0),
+            alt_text: "My Image".to_owned(),
         }
     }
 }
@@ -185,6 +187,11 @@ impl eframe::App for ImageViewer {
             ui.label("Aspect ratio is maintained by scaling both sides as necessary");
             ui.checkbox(&mut self.maintain_aspect_ratio, "Maintain aspect ratio");
 
+            // alt text
+            ui.add_space(5.0);
+            ui.label("Alt text");
+            ui.text_edit_singleline(&mut self.alt_text);
+
             // forget all images
             if ui.button("Forget all images").clicked() {
                 ui.ctx().forget_all_images();
@@ -211,6 +218,9 @@ impl eframe::App for ImageViewer {
                 }
                 image = image.maintain_aspect_ratio(self.maintain_aspect_ratio);
                 image = image.max_size(self.max_size);
+                if !self.alt_text.is_empty() {
+                    image = image.alt_text(&self.alt_text);
+                }
 
                 ui.add_sized(ui.available_size(), image);
             });