Adapt to KPimTextEdit changes

Summary:
This now gives us a KF5::SyntaxHighlighting-based main editor, and thus
for example dark theme support, more precise spell checking areas and
proper style merging between highlighting and spellchecking.

Reviewers: #kde_pim, mlaurent

Reviewed By: #kde_pim, mlaurent

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D7673