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_demo_lib/src/demo/misc_demo_window.rs

@@ -1,4 +1,5 @@
 use super::*;
+
 use egui::*;
 
 /// Showcase some ui code
@@ -154,6 +155,10 @@ impl View for MiscDemoWindow {
                 });
             });
 
+        CollapsingHeader::new("Ui Stack")
+            .default_open(false)
+            .show(ui, ui_stack_demo);
+
         CollapsingHeader::new("Misc")
             .default_open(false)
             .show(ui, |ui| {
@@ -492,6 +497,72 @@ impl Tree {
 
 // ----------------------------------------------------------------------------
 
+fn ui_stack_demo(ui: &mut Ui) {
+    ui.horizontal_wrapped(|ui| {
+        ui.label("The");
+        ui.code("egui::Ui");
+        ui.label("core type is typically deeply nested in");
+        ui.code("egui");
+        ui.label(
+            "applications. To provide context to nested code, it maintains a stack \
+                        with various information.\n\nThis is how the stack looks like here:",
+        );
+    });
+    let stack = ui.stack().clone();
+    Frame {
+        inner_margin: ui.spacing().menu_margin,
+        stroke: ui.visuals().widgets.noninteractive.bg_stroke,
+        ..Default::default()
+    }
+    .show(ui, |ui| {
+        egui_extras::TableBuilder::new(ui)
+            .column(egui_extras::Column::auto())
+            .column(egui_extras::Column::auto())
+            .header(18.0, |mut header| {
+                header.col(|ui| {
+                    ui.strong("id");
+                });
+                header.col(|ui| {
+                    ui.strong("kind");
+                });
+            })
+            .body(|mut body| {
+                for node in stack.iter() {
+                    body.row(18.0, |mut row| {
+                        row.col(|ui| {
+                            let response = ui.label(format!("{:?}", node.id));
+
+                            if response.hovered() {
+                                ui.ctx().debug_painter().debug_rect(
+                                    node.max_rect,
+                                    Color32::GREEN,
+                                    "max_rect",
+                                );
+                                ui.ctx().debug_painter().circle_filled(
+                                    node.min_rect.min,
+                                    2.0,
+                                    Color32::RED,
+                                );
+                            }
+                        });
+
+                        row.col(|ui| {
+                            ui.label(if let Some(kind) = node.kind {
+                                format!("{kind:?}")
+                            } else {
+                                "-".to_owned()
+                            });
+                        });
+                    });
+                }
+            });
+    });
+
+    ui.small("Hover on UI's ids to display their origin and max rect.");
+}
+
+// ----------------------------------------------------------------------------
+
 fn text_layout_demo(ui: &mut Ui) {
     use egui::text::LayoutJob;