Support simplified ListElementNeighbors response and split ElementPath component

Requires backend!2096 (merged), closes backend#1553 (closed), closes #1102 (closed)

On top of handling the updated API, this allows the element header to display everything it can except paths in the event that paths take a while to load, with a loading icon where the paths should be. This means if we ever get into trouble with the performance of ListElementNeighbors, this will have a lower impact, as users can still use the Display and Actions menus.