New feature #8757: Individual subquestion relevance

These four commits
- let survey admins enter individual relevance equations per subquestion for various questions types and save these to the DB (see picture)
- expand the fieldmap in common_helper to hold SQrelevance
- expand EM's data structures to hold SQrelevance and
- write everything to $subQrels[] so the respective JS code for EM is created

![image](https://f.cloud.github.com/assets/4358578/2379584/c39c516a-a89c-11e3-92b2-d171ffdf887c.png)


As I could build on TMSWhite's great work, there were actually not many lines of code necessary. But don't ask how many traceVars I needed to get the hang of EM ;)

This should all play along nicely with array filters, exclusive options, cascading individual subquestion relevance, question relevance etc.

Feel free to come up with better/nicer icons. I tried but I am not an icon designer...