Don't register mimetype for diff editor

Task-number: QTCREATORBUG-11624

Change-Id: I9eeeeeb862e335c1a4d5cbd80d0908297ad6d1db
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Fix messages in QNX.

Change-Id: Id9199607e787a04a3c90596e30e5ca5f249e7f07
Reviewed-by: Tobias Nätterlund <tobias.naetterlund@kdab.com>
Reviewed-by: Mehdi Fekari <mfekari@blackberry.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>