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).