Refactor handling of spell checking

[vkbo]

Summary:

This PR:

• Moves the spell check instance to the new shared data class, so that the object can be destroyed and re-created with the project.
• Cleans up loading of the user's own custom dictionary, which is per-project and must be handled correctly to prevent bleed-through between projects.
• Adds a way to change spell check language from the Tools menu.

Also:

• The project class is no longer a subclass of QObject.
• Various attributes of core classes have been converted to properties.
• The syntax highlighter class has received a cleanup.

Related Issue(s):

Closes #1260

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