Blueprint tree context menu: `Move to container` -> `Tabs` is broken

In the blueprint tree context menu, right-clicking on a view and choosing `Move to container -> Tabs` does nothing most of the time. That's because we always wrap views in tab containers (so they have a title bar), and egui_tiles' GC prunes a tab-in-tab hierarchy.

The root issue here is the requirement to wrap all views in a tab container. egui_tiles should support pane decorations.