Server folder methods

This supersedes #4882 as I fuxxed that up :P

I made an all-purpose getter for folders which is called by getUserFolder/getAppFolder. A bit more flexible and DRY ;)

I can't really test it - other than it doesn't break anything - because filelisting seems to be broken in appframework-master branch. I can only see my Shared folder in the Files app.

@DeepDiver1975 @icewind1991 @karlitschek