[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