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>

crates/egui_plot/src/legend.rs

@@ -250,7 +250,7 @@ impl Widget for &mut LegendWidget {
         let layout = Layout::from_main_dir_and_cross_align(main_dir, cross_align);
         let legend_pad = 4.0;
         let legend_rect = rect.shrink(legend_pad);
-        let mut legend_ui = ui.child_ui(legend_rect, layout);
+        let mut legend_ui = ui.child_ui(legend_rect, layout, None);
         legend_ui
             .scope(|ui| {
                 let background_frame = Frame {

crates/egui_plot/src/lib.rs

@@ -1488,7 +1488,7 @@ impl<'a> PreparedPlot<'a> {
 
         let transform = &self.transform;
 
-        let mut plot_ui = ui.child_ui(*transform.frame(), Layout::default());
+        let mut plot_ui = ui.child_ui(*transform.frame(), Layout::default(), None);
         plot_ui.set_clip_rect(transform.frame().intersect(ui.clip_rect()));
         for item in &self.items {
             item.shapes(&plot_ui, transform, &mut shapes);