Do not send file list for select all on Download/delete

- When all files are selected, do not send the whole file list
- Download will trigger download for the parent folder, also works
  with root
- Delete will send "allfiles" to the server that will find the filelist
  or the passed directory by itself

Note: this is not only a performance improvement but is also mandatory for infinite scrolling (can't select all elements that haven't appeared yet)

Please review @karlitschek @DeepDiver1975 @kabum @schiesbn