Allow always "Assign to next element"

[andre-hohmann]

Is your feature request related to a problem? Please describe.
If "Show only structure elements" is activated in "customize display" for the structure, it is not possible anymore to apply "Assign to next element" for the pages in the structured view of the gallery.
Therefore, it is necessary to switch in between the two states of the structure tree. That is annoying for the users.

Probably, this is connected to the reason, that "Assign to next element" is not possible in the "Extended structure tree".

• Linking pages to multiple structure elements not possible in "Erweiterter Strukturbaum" #4380

Describe the solution you'd like
It should be possible to assign pages to the next element in the structured view of the gallery, even if only structure elements are shown in the structure tree.

Describe alternatives you've considered
If the implementation of the requirement causes too far-reaching adjustments to the software, the current status should be retained.
These use cases probably do not occur so frequently and the resources can be invested in other developments.

Additional context
Activate "Show only structure elements"

Example: "Assign to next element" is not possible

Example: "Assign to next element" is possible