Various improvements

* Improves: performance during upload (for complex folder tree), of the first render and during lifetime
* Fixes: folder with only folders as children was not created
* Fixes: new folder was not sorted properly among folders
* Fixes: upload status was overlapping the content when browsing
* ~~I changed the way deletion works server-side to take benefit from the "remove the request's results" API of the Data System (it saves N-1 HTTP requests and could handle mass deletion in a faster way in the future)~~ Due to a bug in the Data System, I let the original behaviour (with revamped code flow) but will do it again when the DS is fixed.

* Adds: when a folder has files being uploaded in its subtree, its icon spins (close #128).