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 
hm ... The limit I ran into is 1000: https://github.com/owncloud/core/commit/592a2dc82f0d6babd3830292eebd362e04e5c070#diff-fe310df5db90426b42447108ef4d3483L331

The code has since been removed by recursively descending the tree ...