Port to KF5::SyntaxHighlighting

Summary:
This gives us a generic syntax highlighter with spell checking, using the
spell checking areas from the syntax definition files (which are more
precise than the previous line-based approach). The implementation is a
bit messy unfortunately since neither QSyntaxHighlighter nor QTextBlock
are made for this kind of highlighter composition.

Reviewers: KDE PIM, mlaurent

Reviewed By: KDE PIM, mlaurent

Subscribers: mlaurent

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D7672