tvdb id to imdb id translation; remove primary metadata agent dependency

Two modification made:

1. TVDB to IMDB ID translation

Although TVDB is set as primary agent, there is no info used in the code.
So new function TvdbId_to_ImdbId is added to enable OpenSubtitles search feature based on IMDB ID.

2. Primary metadata agent dependency is removed

Searching with IMDB ID is the second priority option in the agent but IMDB & TVDB metadata agent are requested to use OpenSubtitles.
So I commented out 'contributed_to' lines from the code.

I'm also trying to run with other metadata agent that passes IMDB & TVDB ID to help OpenSubtitles search.