Summary:

The primary purpose of this PR is to allow documents and notes in the project tree to have child items. I.e. one can have a chapter document with scene documents as child elements.

This PR changes the following things:

• Change the project file format to save expanded status also for items of type FILE.
• Remove the drag and drop restriction on dropping files on other files.
• Implement recursive item update, which means an item with child items can no be moved.
• Allow moving folders between root folders.
• Completely disable the ability to drag a root folder.
• Drop nearly all of the checking code in the dropEvent implementation of the project tree as either all cases are allowed now, or the few remaining disallowed actions are blocked on a lower level.
• Remove TRASH as an item type. The Trash folder also has a class setting TRASH, so the information is in reality redundant. The Trash folder is now of type ROOT. This simplifies a few checks here and there.

Related Issue(s):

Resolves #1002

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed