Add text zoom in editor and viewer

[vkbo]

Summary:

This PR changes how the heading sizes are handled in the editor and viewer and uses the hardcoded size values in Qt, which are fairly similar to those novelWriter used anyway. This makes the default zoom feature in Qt usable in both editor and viewer.

The zoom options are tied to common shortcuts, and exposed in the View menu. Since Ctrl+0 is the common shortcut for zoom reset, it is remapped to this rather than its old use to reset the format on a text block. Ctrl+Shift+/ still handles the latter like before.

Related Issue(s):

Closes #1280

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 linting 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