Style feature to have label truncate by default

[abey79]

In some context, it would be useful to have ui.label(txt) defaults to truncate, such as we don't have to change them all for ui.add(egui::Label::new(txt).truncate(true)). Likewise, it would be nice to have a ui.label_truncated() helper (though each might be suited for different situations).

As per:

• Use the ListItem style for the component list (in the Selection Panel when an entity is selected) rerun-io/rerun#4161 (comment)

[emilk]

Would ui.style().truncate = Some(true); solve most of the problems @abey79, or do you still want label_trunc and label_wrap?

[abey79]

I think ui.style().truncate = Some(true); would actually solve this more elegantly than dedicated ui.label_trunc. It's also more in-line with the bigger CSS-like styling story.