Create custom egui_kittest::Node (#7138)

This adds a custom Node struct with proper support for egui types
(`Key`, `Modifiers`, `egui::Event`, `Rect`) instead of needing to use
the kittest / accesskit types.

I also changed the `click` function to do a proper mouse move / mouse
down instead of the accesskit click. Also added `accesskit_click` to
trigger the accesskit event. This resulted in some changed snapshots,
since the elements are now hovered.

Also renamed `press_key` to `key_press` for consistency with
`key_down/key_up`.

Also removed the Deref to the AccessKit Node, to make it clearer when to
expect egui and when to expect accesskit types.

* Closes #5705 
* [x] I have followed the instructions in the PR template

crates/egui_demo_app/tests/snapshots/clock.png

@@ -1,3 +1,3 @@
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-size 334792
+oid sha256:0bd688ff74f9a096edab545fbcbf61b61a464183da066ae4a120ce1e2abf3e7b
+size 334969

crates/egui_demo_app/tests/snapshots/custom3d.png

@@ -1,3 +1,3 @@
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-size 92185
+oid sha256:c80c4ae4c2bfbc5c91e9cd94213a4f87646fe910b4a7c747531a1efcf23def47
+size 92364

crates/egui_demo_app/tests/snapshots/easymarkeditor.png

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-size 178887
+oid sha256:8cf6d0b20f127f22d49daefed27fc2d0ca43d645fe1486cf7f6fcbb676bdec82
+size 179065

crates/egui_demo_app/tests/snapshots/imageviewer.png

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+size 100845

crates/egui_demo_app/tests/test_demo_app.rs

@@ -55,7 +55,7 @@ fn test_demo_app() {
                 harness
                     .get_by_role_and_label(Role::TextInput, "URI:")
                     .focus();
-                harness.press_key_modifiers(egui::Modifiers::COMMAND, egui::Key::A);
+                harness.key_press_modifiers(egui::Modifiers::COMMAND, egui::Key::A);
 
                 harness
                     .get_by_role_and_label(Role::TextInput, "URI:")