Implement accessibility APIs via AccessKit (#2294)

* squash before rebase

* Update AccessKit, introducing support for editable spinners on Windows and an important fix for navigation order on macOS

* Restore support for increment and decrement actions in DragValue

* Avoid VoiceOver race condition bug

* fix clippy lint

* Tell AccessKit that the default action for a text edit (equivalent to a click) is to set the focus. This matters to some platform adapters.

* Refactor InputState functions for AccessKit actions

* Support the AccessKit SetValue for DragValue; this is the only way for a Windows AT to programmatically adjust the value

* Same for Slider

* Properly associate the slider label with both the slider and the drag value

* Lazily activate egui's AccessKit support

* fix clippy lint

* Update AccessKit

* More documentation, particularly around lazy activation

* Tweak one of the doc comments

* See if I can get AccessKit exempted from the 'missing backticks' lint

* Make PlatformOutput::accesskit_update an Option

* Refactor lazy activation

* Refactor node mutation (again)

* Eliminate the need for an explicit is_accesskit_active method, at least for now

* Fix doc comment

* More refactoring of tree construction; don't depend on Arc::get_mut

* Override a clippy lint; I seem to have no other choice

* Final planned refactor: a more flexible approach to hierarchy

* Last AccessKit update for this PR; includes an important macOS DPI fix

* Move and document the optional accesskit dependency

* Fix comment typo

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

* reformat

* More elegant code for conditionally creating a node

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

* Set step to 1.0 for all integer sliders

* Add doc example for Response::labelled_by

* Clarify a TODO comment I left for myself

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

crates/epaint/src/text/cursor.rs

@@ -113,7 +113,7 @@ impl PartialEq for PCursor {
 /// They all point to the same place, but in their own different ways.
 /// pcursor/rcursor can also point to after the end of the paragraph/row.
 /// Does not implement `PartialEq` because you must think which cursor should be equivalent.
-#[derive(Clone, Copy, Debug, Default)]
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct Cursor {
     pub ccursor: CCursor,