Fix fetching topics, moved to an internal search API

[benoit74]

Fix #149

Changes

• move to an internal search API to find videos per topic
• enhance the utility method download_link to also perform POST requests needed by the search
• rewrite the filtering by language now that it is not available at search-time anymore when searching by topic
• topic names must not be URL-escaped anymore
• explicitly use the URL path to deduplicate videos (used only for playlists now)

Nota: code quality is obviously not optimal, I will fix it all at once with #148 which will be done right after this is merged.

[rgaudin]

Thank you ; please add a docstring to request_url since it now has two behaviors and we're good to merge.