Add (reference) files after task creation

To realize issue https://jira.translate5.net/browse/T5CONNECT-23 it will be necessary to add a (pdf) file to an existing task. The easiest way will be to add this file as a reference file within a defined folder.
To do this, the translate5 API must be improved so reference files can be added AFTER creating a task.
It may also be useful to delete a (reference) file. So this may also should get a though on developing this issue.