Rehighlight tags on note creation

[vkbo]

Summary:

This PR adds a new slot in the document editor to listen to changes in tags from the index and updates the syntax highlighting on all meta data lines. This ensures that when a new note is created from an unknown reference in the editor, the highlighting changes afterwards.

Related Issue(s):

Closes #1916

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