Add outline to manuscript tool

[vkbo]

Summary:

This PR adds an outline of the preview document to the Manuscript Build Tool. The build details and outline are occupying the same area of the dialog, under different tabs.

The manuscript preview now also used the HTML class in preview mode, like the document viewer. This simplifies a few things, and makes it more consistent with the output. In addition, the HTML class now generates navigation links for all the titles using the handle as part of the anchor. This will both be useful when multi-document viewing is added for the document viewer, and table of contents navigation is added to the HTML output. The old anchors only used a heading counter to generate the link, which resets on each included document. The navigation for the document viewer has been updated to account for the new anchors.

In addition, this PR combines all the tool button styling into a central style sheet dictionary in the main theme class. The specially styled tool button used all over the GUI has also been combined into a subclass of the QToolButton so that the styling doesn't have to be set everywhere this button style is used. This means that all tool buttons on the GUI using this style now have the same look. Before this, there were minor variations.

Related Issue(s):

Closes #1765

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