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_extras/src/datepicker/button.rs

@@ -140,6 +140,7 @@ impl<'a> Widget for DatePickerButton<'a> {
                 inner: saved,
                 response: area_response,
             } = Area::new(ui.make_persistent_id(self.id_source))
+                .kind(egui::UiKind::Picker)
                 .order(Order::Foreground)
                 .fixed_pos(pos)
                 .show(ui.ctx(), |ui| {

crates/egui_extras/src/layout.rs

@@ -195,9 +195,12 @@ impl<'l> StripLayout<'l> {
         child_ui_id_source: egui::Id,
         add_cell_contents: impl FnOnce(&mut Ui),
     ) -> Ui {
-        let mut child_ui =
-            self.ui
-                .child_ui_with_id_source(rect, self.cell_layout, child_ui_id_source);
+        let mut child_ui = self.ui.child_ui_with_id_source(
+            rect,
+            self.cell_layout,
+            child_ui_id_source,
+            Some(egui::UiStackInfo::new(egui::UiKind::TableCell)),
+        );
 
         if flags.clip {
             let margin = egui::Vec2::splat(self.ui.visuals().clip_rect_margin);