Introduce UiStack (#4588)

* Closes #4534

This PR:
- Introduces `Ui::stack()`, which returns the `UiStack` structure
providing information on the current `Ui` hierarchy.
- **BREAKING**: `Ui::new()` now takes a `UiStackInfo` argument, which is
used to populate some of this `Ui`'s `UiStack`'s fields.
- **BREAKING**: `Ui::child_ui()` and `Ui::child_ui_with_id_source()` now
take an `Option<UiStackInfo>` argument, which is used to populate some
of the children `Ui`'s `UiStack`'s fields.
- New `Area::kind()` builder function, to set the `UiStackKind` value of
the `Area`'s `Ui`.
- Adds a (minimalistic) demo to egui demo (in the "Misc Demos" window).
- Adds a more thorough `test_ui_stack` test/playground demo.

TODO:
- [x] benchmarks
- [x] add example to demo

Future work:
- Add `UiStackKind` and related support for more container (e.g.
`CollapsingHeader`, etc.)
- Add a tag/property system that would allow adding arbitrary data to a
stack node. This data could then be queried by nested `Ui`s. Probably
needed for #3284.
- Add support to track columnar layouts.

---------

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

tests/test_viewports/src/main.rs

@@ -457,7 +457,7 @@ fn drop_target<R>(
 
     let available_rect = ui.available_rect_before_wrap();
     let inner_rect = available_rect.shrink2(margin);
-    let mut content_ui = ui.child_ui(inner_rect, *ui.layout());
+    let mut content_ui = ui.child_ui(inner_rect, *ui.layout(), None);
     let ret = body(&mut content_ui);
 
     let outer_rect =