Cross-widget text select (#3870)

* Closes https://github.com/emilk/egui/issues/3816


![cross-widget-text-selection](https://github.com/emilk/egui/assets/1148717/5582b9d2-8b04-47b7-9637-f48e44064a70)

Turn off with `style.interaction.multi_widget_text_select`.

There is an API for this in `LabelSelectionState`, but it's pretty
bare-bones.

This became really hairy implementation-wise, but it works decently
well.

# Limitations
* Drag-select to scroll doesn't work
* A selection disappears if you scroll past one of its end-points
* Only the text of labels and links are selectable
 
## TODO
* [x] An option to turn it off
* [x] An API for querying about the selected text, and to deselect it.
* [x] Scrolling past selection behaves weird
* [x] Shift-click to select a range

crates/egui_demo_lib/src/demo/demo_app_windows.rs

@@ -32,6 +32,7 @@ impl Default for Demos {
             Box::<super::MiscDemoWindow>::default(),
             Box::<super::multi_touch::MultiTouch>::default(),
             Box::<super::painting::Painting>::default(),
+            Box::<super::panels::Panels>::default(),
             Box::<super::plot_demo::PlotDemo>::default(),
             Box::<super::scrolling::Scrolling>::default(),
             Box::<super::sliders::Sliders>::default(),
@@ -42,7 +43,6 @@ impl Default for Demos {
             Box::<super::widget_gallery::WidgetGallery>::default(),
             Box::<super::window_options::WindowOptions>::default(),
             Box::<super::tests::WindowResizeTest>::default(),
-            Box::<super::window_with_panels::WindowWithPanels>::default(),
         ])
     }
 }

crates/egui_demo_lib/src/demo/mod.rs

@@ -19,6 +19,7 @@ pub mod misc_demo_window;
 pub mod multi_touch;
 pub mod paint_bezier;
 pub mod painting;
+pub mod panels;
 pub mod password;
 pub mod plot_demo;
 pub mod scrolling;
@@ -31,7 +32,6 @@ pub mod text_layout;
 pub mod toggle_switch;
 pub mod widget_gallery;
 pub mod window_options;
-pub mod window_with_panels;
 
 pub use {
     about::About, demo_app_windows::DemoWindows, misc_demo_window::MiscDemoWindow,

crates/egui_demo_lib/src/demo/panels.rs

@@ -1,15 +1,15 @@
 #[derive(Clone, Default, PartialEq, Eq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
-pub struct WindowWithPanels {}
+pub struct Panels {}
 
-impl super::Demo for WindowWithPanels {
+impl super::Demo for Panels {
     fn name(&self) -> &'static str {
-        "🗖 Window With Panels"
+        "🗖 Panels"
     }
 
     fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
         use super::View as _;
-        let window = egui::Window::new("Window with Panels")
+        let window = egui::Window::new("Panels")
             .default_width(600.0)
             .default_height(400.0)
             .vscroll(false)
@@ -18,7 +18,7 @@ impl super::Demo for WindowWithPanels {
     }
 }
 
-impl super::View for WindowWithPanels {
+impl super::View for Panels {
     fn ui(&mut self, ui: &mut egui::Ui) {
         // Note that the order we add the panels is very important!