Allow to move multiple page at once in the Structure tree

[BartChris]

Is your feature request related to a problem? Please describe. Right now it is possible to select multiple pages in the gallery view using shift+click. One can then drag those pages to structures in the Gallery. The selection however is not reflected in the structure tree. It is also not possible to select multiple pages in the structure tree and drag them to structural elements in the structure tree.

Describe the solution you'd like It should be possible to select multiple pages in the structure tree and to drag multiple pages to a structural element. The selections should be reflected in the Gallery. Selecting multiple pages in the gallery should be reflected in the structure tree.

[solth]

Closely related to, if not even a duplicate of #3629