Save both status and importance flags

[vkbo]

Summary:

In the old project file format, the status and importance flags were saved to the same setting in the project file, and interpreted as status or importance based on the class. This meant that if an item is moved from one class folder to another where the interpretation changes, and then back again, the setting is lost.

With the new file format, saving both is less of an issue, so the old setting is now preserved as long as the status or importance entry still exists in the project.

This PR also moves the handling of the status/importance icons from the main GUI class into the NWStatus class itself. This makes more sense, but was not done in the past in order to keep QtGui elements out of the novelWriter core subpackage. Since the issue there was mostly with testing, a quick hack has been added to the conftest.py file.

Related Issue(s):

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