Make MenuLists populate their contents when opened, not beforehand

Sending the full MenuList contents each time a layout is sent seems to be causing massive performance slowdowns and memory allocations that have to later be GC'd. We want a MenuList to request its contents from the backend when it's opened, rather than being sent ahead of time in the layout that contains the MenuList. Profiling showed that MenuList creation was one cause of the editor being very slow (back when we were using Vue and its virtual DOM, at least— we'd need to re-profile it under Svelte to get a proper point of comparison).

Empieza trazando cómo se incluyen los contenidos de MenuList en el layout y cómo el backend puede proporcionarlos cuando se abre un menú. Perfila el flujo actual de creación y layout, y después verifica que las MenuLists que no se han abierto ya no envíen todos sus contenidos y que, al abrir una, sus contenidos se sigan cargando correctamente.

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