since the async highlighter implementation the parent is not a
TextDocument anymore, but a QTextDocument. Pass the required mimeType
via the SyntaxHighlighterRunner now to the SyntaxHighlighter.
Change-Id: I0afcbe68719195649b4b7040416a743cb01214df
Reviewed-by: Artem Sokolovskii <artem.sokolovskii@qt.io>