We have several implementations of the Re/Fwd prefix stripping for subjects, in messagecore (StringUtil::stripOffPrefix), messageviewer (NodeHelper::cleanSubject) and messagecomposer (MessageHelper::cleanSubject). Prefix replacement (for replying/forwarding) also exists in messageviewer and messagecomposer, and the settings seem to be also in at least two places.

Unify all this in a single place in messagecore, there does not seem to be a reason for different behavior, also the current one does not seem entirely consistent (regarding considering settings and i18n for example).