* closes #5674

This changes egui to create an AccessKit node for each `Ui`. I'm not
sure if this alone will directly improve accessibility, but it should
make it easier to create the correct parent / child relations (e.g.
grouping menus as children of menu buttons).
Instead of having a global stack of parent ids, they are now passed via
a parent_id field in `UiBuilder`.

If having all these `GenericContainer` nodes somehow is bad for
accessibility, the PR could also be changed to only create nodes if
there is actually some accessibility info with it (the relevant is
currently commented-out in the PR). But I think screen readers should
just ignore these nodes, so it should be fine? We could also use this as
motivation to git red of some unnecessary wrapped `Ui`s, e.g.
CentralPanel creates 3 Uis when 2 should be enough (the initial Ui and a
Frame, maybe we could even only show the `Frame` if we can give it an
UiBuilder and somehow show the Frame with `Ui::new`).

Here is a screenshot from the accessibility inspector
(#7368) with this PR:

<img width="431" height="744" alt="Screenshot 2025-07-24 at 12 09 55"
src="https://github.com/user-attachments/assets/6c4e5ff6-5c38-450e-9500-0776c9018d8c"
/>

Without this PR:


https://github.com/user-attachments/assets/270e32fc-9c7a-4dad-8c90-7638c487a602

Adds an accessibility inspector plugin that shows the current AccessKit
tree:


https://github.com/user-attachments/assets/78f4f221-1bd2-4ce4-adf5-fc3b00f5c16c

Macos has a built in accessibility inspector, but it doesn't seem to
work with AccessKit / eframe so this provides some insight into the
accesskit state.

This also showed a couple issues that are easy to fix:
- [ ] Links show up as `Label` instead of links
- [ ] Not all supported actions are advertised (e.g. scrolling)
- [ ] The resize handles in windows shouldn't be focusable
- [ ] Checkbox has no value
- [ ] Menus should have the button as parent widget (not 100% sure on
this one)


Currently the plugin lives in the demo app, but I think it should be
moved somewhere else. Maybe egui_extras?

This could also be relevant for emilk#4650

* closes emilk#5674

This changes egui to create an AccessKit node for each `Ui`. I'm not
sure if this alone will directly improve accessibility, but it should
make it easier to create the correct parent / child relations (e.g.
grouping menus as children of menu buttons).
Instead of having a global stack of parent ids, they are now passed via
a parent_id field in `UiBuilder`.

If having all these `GenericContainer` nodes somehow is bad for
accessibility, the PR could also be changed to only create nodes if
there is actually some accessibility info with it (the relevant is
currently commented-out in the PR). But I think screen readers should
just ignore these nodes, so it should be fine? We could also use this as
motivation to git red of some unnecessary wrapped `Ui`s, e.g.
CentralPanel creates 3 Uis when 2 should be enough (the initial Ui and a
Frame, maybe we could even only show the `Frame` if we can give it an
UiBuilder and somehow show the Frame with `Ui::new`).

Here is a screenshot from the accessibility inspector
(emilk#7368) with this PR:

<img width="431" height="744" alt="Screenshot 2025-07-24 at 12 09 55"
src="https://github.com/user-attachments/assets/6c4e5ff6-5c38-450e-9500-0776c9018d8c"
/>

Without this PR:

https://github.com/user-attachments/assets/270e32fc-9c7a-4dad-8c90-7638c487a602