Use the workbox interface

[tbttfox]

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)

Proposed Changes

In two places, the scintilla interface was used instead of the WorkboxMixin interface.
QTextEdit and QPlainTextEdit don't have .lines or .text methods