Update the novel tree GUI

[vkbo]

Summary:

Update the Novel Tree look and functionality to match the Project Tree and Outline View:

• The header is gone and replaced with a stretching title column, a resize to fit word count column, and a resize to fit optional third column.
• The third column can be selected to be hidden, shot point of view character, focus character, or plot tag.
• The novel root folder to be shown is selected from a toolbutton menu.

Related Issue(s):

Closes #1041

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