Deleting all files in trash now only sends a single flag

To prevent having to send the list of all files for deletion, only set a
flag "allfiles". This should make it a bit smoother when deleting 5000+
files.

Also fixes some "empty trash" message issues.
Merge pull request #5641 from owncloud/filestrash-emptytrash

Deleting all files in trash now only sends a single flag