Potential premature displaying of 1-movie-sets results

[wutschel]

When ignoring movie sets with only 1 movie there is potential problem which could cause premature display of the results. There are 2 embedded/nested JSON requests placed. The code expects the inner JSON responses to arrive in the same order as the requests were placed. Due to the asynchronous nature this is not guaranteed -- use of dispatchGroup (triggering result's display within notify block) was suggested in https://github.com/xbmc/Official-Kodi-Remote-iOS/pull/597.