added support for bookmark tools

Bookmark tools folders support added to the bookmarks form, following
exactly the same pattern as for file_tools. Not tested on a live site but appears to be working fine.
We created bookmark tools based on file tools in order to allow bookmarks to be organized into folders - available at https://github.com/AU-Landing-Project/Bookmark-Tools (and I'll try to persuade Matt to add it to the community site as it is mighty useful!)
Jon