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_kittest/tests/regression_tests.rs

@@ -1,4 +1,4 @@
-use egui::Button;
+use egui::{Button, Image, Vec2, Widget};
 use egui_kittest::{kittest::Queryable, Harness};
 
 #[test]
@@ -27,3 +27,19 @@ pub fn focus_should_skip_over_disabled_buttons() {
     let button_1 = harness.get_by_label("Button 1");
     assert!(button_1.is_focused());
 }
+
+#[test]
+fn image_failed() {
+    let mut harness = Harness::new_ui(|ui| {
+        Image::new("file://invalid/path")
+            .alt_text("I have an alt text")
+            .max_size(Vec2::new(100.0, 100.0))
+            .ui(ui);
+    });
+
+    harness.run();
+    harness.fit_contents();
+
+    #[cfg(all(feature = "wgpu", feature = "snapshot"))]
+    harness.wgpu_snapshot("image_snapshots");
+}