FR: Directionless / internal items

Though Docspell seems to have originally been targeted at handling various types of correspondence, I've found Docspell to be a very useful way to store essentially anything that can be put on paper. This is especially the case when I'm working on a report or similar - I sometimes create supporting notes and spreadsheets and such, and it's useful to keep these in Docspell and link them to the actual document. It would be nice to have a first-class concept of an internal/directionless file, for files created by me for me, since currently I tag these as being outgoing files to no correspondent.