[WIP] File list sorting

:construction: 

Work in progress. Requires the "files-ajaxload" branch to be merged first.

This PR will contain file list sorting.
- [x] Sortable file list
- [ ] Sortable trashbin
- [ ] Sortable public page
- [ ] Upload file while sorted must appear at the correct position
- [ ] Sort indicator on headers (arrows?)
- [ ] Clean up server-side comparators ?