Fix font style and PDF issues

[vkbo]

Summary:

This PR fixes various font issues related to the document viewer, manuscript preview, and PDF output. These all use the same ToQTextDocument converter class. They have various font-related issues. The cause was the font matching algorithm in Qt5 being lazy and replacing fonts when changes were made in Preferences, and letting them replace the chose build settings font. A helper function now tries to overcome this, if it can. It seems to work,

This PR also fixes some minor issues in the ODT output, and fixes a bug related to resetting to regular/normal font style when using emphasis in the text, instead of the chosen default style. This only affected the ToQTextDocument converter.

Related Issue(s):

Closes #2121
Closes #2118
Related #2110

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