Add a toolbar for main GUI "views"

[vkbo]

Summary:

This PR adds a toolbar on the left margin of the screen. There are two buttons that replace the Project / Novel tabs under the project tree, and a button that switches to Outline view. The three toolbar buttons under the project tree have also been moved to the bottom of this new toolbar.

This redesign uses slightly more horizontal space than the old layout, but only a few pixels. The toolbar is a bit wider than the tabs that were removed on the right side. However, the toolbar looks a bit more modern than the tabs. It also more flexible as the buttons can be used in a few more ways than a plain tab can. More features can be moved here for quick access.

Related Issue(s):

Resolves #1056

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