Improve spell checking

[vkbo]

Summary:

This PR:

• Improves the spell checker to use the finditer method of pattern object directly. This method allows passing an offset value, which is needed in some places.
• Adds an "Ignore Word" entry to the text editor context menu. This will ignore the word for the duration of the spell checking session, but not save it to the project dictionary.

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