Lun 1169 -- filer - cropduster integration

If the folder admin is opened with the size_set_id in the request parameters, it has open cropduster admin windows after an image was selected by the user.

In folderadmin.py (directory_listing method), it checks for both size_set_id and cropduster_url. The size_set_id has to be stored in the session in order to make it available when the user browse the folders.