[text thumbnailer] Use KSyntaxHighlighting for text rendering
Summary:
Makes previews of text files a bit easier to recognize, both due to the
highlighting helping with reading as well as color pattern adding to
recognizability of files by their preview.
Test Plan: Renders fine for any kind of text/plain MIME types and sub formats.
Reviewers: broulik, cfeck
Reviewed By: cfeck
Subscribers: vkrause, cfeck, kde-frameworks-devel, kfm-devel
Tags: Dolphin, Frameworks
Differential Revision: https://phabricator.kde.org/D19432