Improve datastore browser column header UI

[abey79]

What

Uses the new egui::Sides API to display the sort arrow. Note that the truncation behaviour is not quite perfect yet.

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[emilk]

I would expect this arrow to be clickable

[abey79]

Makes sense.

Interestingly, this brings up a small paper cut with the current Sides api: I can't mutably borrow anything in both closure, so I'm forced to do something like this:

    let mut toggle_left = false;
    let mut toggle_right = false;

    egui::Sides::new()
        .height(re_ui::DesignTokens::table_line_height())
        .show(
            ui,
            |ui| {
                ui.style_mut().wrap_mode = Some(egui::TextWrapMode::Truncate);

                toggle_left = ui.button(egui::WidgetText::from(label).strong()).clicked();
            },
            |ui| {
                toggle_right = ui
                    .button(match (is_sorted, direction) {
                        (true, SortDirection::Ascending) => "↓",
                        (true, SortDirection::Descending) => "↑",
                        _ => "",
                    })
                    .clicked();
            },
        );

At a minimum, Sides::show() should return a tuple of (R1, R2) generic types corresponding to the return value of each closure.

[abey79]

emilk/egui#5065