Add Label text selection api

[mankinskin]

• Closes Add access to selected char range on selectable labels #7542
• I have followed the instructions in the PR template

let label = ui.add(Label::new(&text).selectable(true));
if let Some(range) = label.selected_char_range() {
    println!("Selected char range: {:?}", range);
}
label

These changes were generated by Claude AI.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7541-labelselectedcharrange
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

Sorry this is very conflicted now that I've merged the new plugin trait. You'll have to update it to fetch the label selection state via the plugin helpers

[emilk]

Cool!

It would be nice if the egui_demo_lib used this new functionality.

Also: please add a kittest test of it 🙏

[mankinskin]

I still need to find a way to get selections across multiple widgets..

So far I was only able to get the latest selection in the current Context per Viewport. For my app I need to be able to fetch all current selections, across any widgets in the current context and viewport.

I will look at the new plugin api and go from there.

[mankinskin]

I reworked the api a bit and added an example project as a demo.

I have added a map in LabelSelectionState from widget_id: Id to Arc<Galley> to be able to calculate the selected char range on Responses without providing the galley.

I am thinking about storing an entire WidgetSelectionState in this map, to store secondary_reached: bool and primary_reached: bool at render time for each widget.

Otherwise the range calculation will be wrong if the calls to widget_cursor_range on a Response come after another widget has been rendered (because they all share the LabelSelectionState and update primary_reached, ...).

The example shows how to do it correctly at the moment, by extracting the cursor_range in the same iteration the Label is rendered.