Make sure free form text boxes in Preferences are safe

[vkbo]

Summary:

This PR adds a new function that checks free form text box settings in Preferences, and adds constraints to others.

• Narrator break, which can only be one character, has white space stripped,
• Dialog symbol, which can only be one character, has white space stripped.
• Pad before and after symbol box content is now processed so that all white spaces are stripped, and all characters listed are unique. They are also sorted, because the filter uses set, which may return different order on each call.

Related Issue(s):

Closes #1985

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