Add getTagsForObjects in ITags

Returns the list of tags that are set on the given object ids.

Required for the "favorites/tags" task (#2368)  but also a nice addition to the API.
See https://github.com/owncloud/core/pull/12360/files#diff-471be045dd1eb605d9a1197b9ba90560R52

Please review @ockham @LukasReschke @icewind1991 @DeepDiver1975 
So I'll update the PHPDocs instead.