Find in Files Improvements (Part 2: Pagination)

This is the second incremental change from #3151 which mainly only adds Pagination and Double clicking on a file to add it to the working set.

Each page now always shows 100 results as before, and the new links `More` and `Less` allow to see the next results or the previous ones. This can be changed if required. I also incremented the maximum amount of results per file to 300 since we can now see more than 100 results, but this number was randomly chosen so it can be modified. The collapsed folders are also restored when switching pages.