Allow multiple roots of same class

[vkbo]

Summary:

This PR allows each class of root folders to be added multiple times. For notes, this is trivial as no part of the code cares about what root folder a note is in, only what class the folder is. However, for novel documents, the outline and novel tree treats them all as one, which defeats the purpose of allowing multiple novels in one project.

To solve this, the items now track what root folder they belong to. This more or less renders the class setting redundant as it is determined by the root folder, but it is kept anyway because it is faster to look up directly than via the root folder handle. However, the synchronisation of these settings are updated each time an item is changed as well as at load time.

This PR also simplifies the creation of new items, and the updating of settings when an item is moved. A lot more decisions are now handled by the NWItem class itself. Most decisions are now taken in a central place instead of relying on a lot of in-place checks where changes to items are made. This significantly simplifies the logic in the project tree class for instance.

Due to these changes, several restrictions on drag and drop have been lifted. The remaining rules are:

• Files can be moved anywhere, but they are converted to a Note if the root folder does not allow novel documents.
• Folders can only be moved within the same root folder, but can be moved otherwise freely.
• Root folders cannot be moved at all by drag and drop, but can still be moved from the menu.
• Items cannot be dropped on top of a file (moved inside), as before.

Related Issue(s):

Resolves #967

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