Add egui::Scene for panning/zooming a Ui (#5505)

This is similar to `ScrollArea`, but:
* Supports zooming
* Has no scroll bars
* Has no limits on the scrolling

## TODO
* [x] Automatic sizing of `Scene`s outer bounds
* [x] Fix text selection in scenes
* [x] Implement `fit_rect`
* [x] Document / improve API

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

crates/egui_demo_lib/src/demo/mod.rs

@@ -21,9 +21,9 @@ pub mod modals;
 pub mod multi_touch;
 pub mod paint_bezier;
 pub mod painting;
-pub mod pan_zoom;
 pub mod panels;
 pub mod password;
+pub mod scene;
 pub mod screenshot;
 pub mod scrolling;
 pub mod sliders;