Add support for scrolling via accesskit / kittest (#7286)

I need to scroll in a snapshot test in my app, and kittest had no
utilities for this. Event::MouseWheel is error prone. This adds support
for some accesskit scroll actions, and uses this in kittest to add
helpers to scroll to a node / scroll the scroll area surrounding a node.

The accesskit code says down/up/left/right `Scrolls by approximately one
screen in a specific direction.`. Unfortunately it's difficult to get
the size of a "screen" (I guess that would be the size of the containing
scroll area)where I implemented the scrolling, so for now I've hardcoded
it to 100px. I think scrolling a fixed amount is still better than not
scrolling at all.

---------

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

crates/egui_kittest/src/node.rs

@@ -159,4 +159,49 @@ impl Node<'_> {
     pub fn is_focused(&self) -> bool {
         self.accesskit_node.is_focused()
     }
+
+    /// Scroll the node into view.
+    pub fn scroll_to_me(&self) {
+        self.event(egui::Event::AccessKitActionRequest(ActionRequest {
+            action: accesskit::Action::ScrollIntoView,
+            target: self.accesskit_node.id(),
+            data: None,
+        }));
+    }
+
+    /// Scroll the [`egui::ScrollArea`] containing this node down (100px).
+    pub fn scroll_down(&self) {
+        self.event(egui::Event::AccessKitActionRequest(ActionRequest {
+            action: accesskit::Action::ScrollDown,
+            target: self.accesskit_node.id(),
+            data: None,
+        }));
+    }
+
+    /// Scroll the [`egui::ScrollArea`] containing this node up (100px).
+    pub fn scroll_up(&self) {
+        self.event(egui::Event::AccessKitActionRequest(ActionRequest {
+            action: accesskit::Action::ScrollUp,
+            target: self.accesskit_node.id(),
+            data: None,
+        }));
+    }
+
+    /// Scroll the [`egui::ScrollArea`] containing this node left (100px).
+    pub fn scroll_left(&self) {
+        self.event(egui::Event::AccessKitActionRequest(ActionRequest {
+            action: accesskit::Action::ScrollLeft,
+            target: self.accesskit_node.id(),
+            data: None,
+        }));
+    }
+
+    /// Scroll the [`egui::ScrollArea`] containing this node right (100px).
+    pub fn scroll_right(&self) {
+        self.event(egui::Event::AccessKitActionRequest(ActionRequest {
+            action: accesskit::Action::ScrollRight,
+            target: self.accesskit_node.id(),
+            data: None,
+        }));
+    }
 }