Adding basement for the direct download url

refs https://github.com/owncloud/core/issues/12286

@dragotin @danimo @guruz @ogoffart @karlitschek @icewind1991 THX

- [ ] rename dav property to avoid unexpected behavior of client 1.7.x series
Which leaves the client with insufficient info to download the file...

Maybe remove this implementation for now so we can get the majority of the PR merged and discuss this seperately