Fixed issue: repeatheadings didn't work with array_filter

Dev: repeated headings for array questions are generated as static HTML. In a case where an array question has an array filter, all the repeated headings are displayed even if no questions are visible.
Dev: added a javascript function updateHeadings and hooked it in
LEMrelXX in order to dynamically update array headings.

Before patch :
![before](https://f.cloud.github.com/assets/3476089/1390007/f01d4902-3be5-11e3-817a-ee3f9c3bf4f9.png)
After patch :
![after](https://f.cloud.github.com/assets/3476089/1390008/f3225cb4-3be5-11e3-9bcb-d717668f678a.png)