Highlighter: Remove hardcoded colors in doxygen.

Use colors from the creator settings instead.

Task-number: QTCREATORBUG-12420
Change-Id: I477d16420a6e5d99551bb132da71ab5b28a3ff78
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>