Enable multiselect in structure trees

[solth]

The structure trees in the metadata editor currently only support single select. Therefor, synchronization between the gallery and the structure trees gets lost when selecting multiple pages in the gallery.

To fix this, multiselect for pages should be enabled in the structure tree as well, preferably unifying the data sources for all three panels in the editor to simplify and streamline the system.

[andre-hohmann]

The following discussion may be relevant/interesting:

• #5675