New feature #8757: Individual subquestion relevance

For description and discussion of this PR refer to https://github.com/LimeSurvey/LimeSurvey/pull/177.
(same same but different branch)

No, if I remember correctly the problem was that in the DB there was no field for question relevance for questions with subquestions (each subquestion is one question in the DB). Question relevance was saved in the "relevance" field of the last subquestion of the respective question. I had to modify the fieldmap so it could hold relevance and SQ relevance at the same time. Sorry, hard to explain....please test it, should work fine!