Introduce UiStack

[abey79]

• Closes Store a Ui/Frame-stack #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:

•  benchmarks
• 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 Uis. Probably needed for CSS-like styling #3284.
• Add support to track columnar layouts.

[abey79]

benchmark for main, "change" are w.r.t this PR. TL;DR: nothing significant.

demo_with_tessellate__realistic
                        time:   [162.71 µs 163.43 µs 164.22 µs]
                        change: [-0.5310% -0.0758% +0.3821%] (p = 0.75 > 0.05)
                        No change in performance detected.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

demo_no_tessellate      time:   [100.03 µs 100.27 µs 100.55 µs]
                        change: [-0.9036% -0.4831% +0.0081%] (p = 0.04 < 0.05)
                        Change within noise threshold.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high severe

demo_only_tessellate    time:   [63.612 µs 63.759 µs 63.910 µs]
                        change: [+0.0594% +0.6954% +1.3806%] (p = 0.04 < 0.05)
                        Change within noise threshold.
Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) high mild
  1 (1.00%) high severe

label &str              time:   [553.38 ns 618.15 ns 757.60 ns]
                        change: [-18.927% -3.9587% +12.659%] (p = 0.71 > 0.05)
                        No change in performance detected.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high severe

label format!           time:   [607.36 ns 723.32 ns 973.61 ns]
                        change: [-34.326% -6.3731% +32.513%] (p = 0.78 > 0.05)
                        No change in performance detected.
Found 10 outliers among 100 measurements (10.00%)
  6 (6.00%) high mild
  4 (4.00%) high severe

Painter::rect           time:   [31.272 ns 31.625 ns 31.942 ns]
                        change: [-4.8340% -3.4710% -2.1191%] (p = 0.00 < 0.05)
                        Performance has improved.

text_layout_uncached    time:   [62.191 µs 62.366 µs 62.584 µs]
                        change: [+0.1487% +0.3898% +0.6197%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 7 outliers among 100 measurements (7.00%)
  1 (1.00%) low mild
  2 (2.00%) high mild
  4 (4.00%) high severe

text_layout_cached      time:   [209.13 ns 209.47 ns 209.86 ns]
                        change: [-0.0661% +0.1518% +0.3733%] (p = 0.17 > 0.05)
                        No change in performance detected.
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild

tessellate_text         time:   [8.3576 µs 8.4386 µs 8.5532 µs]
                        change: [+0.0148% +0.6121% +1.5481%] (p = 0.10 > 0.05)
                        No change in performance detected.
Found 16 outliers among 100 measurements (16.00%)
  1 (1.00%) low mild
  8 (8.00%) high mild
  7 (7.00%) high severe

[emilk]

Looks good!

[emilk]

If you use references (and lifetimes) we can avoid this .clone(). I think this is likely to be on the hot path of quite a few layout thing in the future.