Support for appending fileRoot in url

This is similar to "expandedFolder" parameter in URL, but it makes such folder as "root".

Use case:
We can have root folder "uploads/", which contains folders "files", "images", "videos"
Then we can work only with "videos", so we can use in url "?fileRoot=videos/" and we will see only content of that folder  + when we choose some file, we could get relative path.

Btw change on line #480 (from !=  to  !==) is required to allow empty relPath - I think this should be fixed anyway :)