Allow to link page to next element independently of hierarchy level

[thomaslow]

Fixes #5457

This pull request allows to add a new link (context menu entry "Assign to next element") for the currently selected page to the following folder independently of its hierarchy level. The selection logic works as follows:

1. find a parent folder whose next sibling is also a folder (continue with parent of parent, if not found)
2. select the deepest child folder of the next sibling only if they are the respective first children

This pull request also checks if a link has already been added before such that users cannot accidentally add the same link twice (the menu entry "Assign to next element" disappears).

Demo

2024-10-08.14-56-08.mp4