Fix infinite recursion when putting a container inside a viewer tab

[emilk]

What

• Closes Viewer hangs upon rearranging tiles into view with tabs #3527

In egui_tiles, not all tiles are panes with space views - some are containers, with more tiles in them.

The existing code on main tried to handle the latter by calling the trait default implementation of a function:

    fn tab_ui(
        &mut self,
        tiles: &egui_tiles::Tiles<SpaceViewId>,
        ui: &mut egui::Ui,
        id: egui::Id,
        tile_id: egui_tiles::TileId,
        active: bool,
        is_being_dragged: bool,
    ) -> egui::Response {
        let Some(tab_widget) = TabWidget::new(self, ui, tiles, tile_id, active, 1.0) else {
            return egui_tiles::Behavior::<SpaceViewId>::tab_ui(
                self,
                tiles,
                ui,
                id,
                tile_id,
                active,
                is_being_dragged,
            );
        };

        …
    }

…the problem is that this is a recursive call, leading to infinite recursion (and the compiler isn't catching it for whatever reason).

The new code in this PR explicitly handles groups, giving them an icon and a name:

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

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[Wumpf]

Nicely done! Sadly the selection behavior doesn't work out for this yet:

Screen.Recording.2023-09-29.at.11.02.17.mov

and the compiler isn't catching it [endless recursion] for whatever reason

well, the recursion wasn't obviously unconditional since it was in the else branch of a condition :)

[Wumpf]

just noticed that the selection behavior I'm complaining was the same in your the description's clip. Somehow it only became unnerving to me when using it....

Anyways, I think the tab container should be greyed out as-if unselected and clicking it should clear the selection instead of keeping the selection on a potentially invisible tab.

[Wumpf]

actually this is - unlike the "always grey" suggestion I made - perfect now 👍

[Wumpf]

cargo deny failure is a known issue