Improve bulk import option so that children are included at once

[emanuelaepure10]

Please follow the template below and provide at least information for the first 4 sections

Background and Motivation

Would be nice to include in the bulk import all the item of the same itemclass with the relations includes, such as parent-child item, item with relation to other register and all in the same file. So the effort for the end user is minimum just to create the import file.

Proposed change

Change the bulk import to accept item of the same item class but of different collections.