Remove t-param parsing from the YouTube plugin

This limits us to displaying videos in the formats listed here:
http://code.google.com/apis/youtube/2.0/reference.html#formatsp. This change
cannot be avoided, as by parsing YouTube video pages, Totem was breaking the
site's terms of service (http://www.youtube.com/t/terms, §5.C).

All URI resolution for YouTube video playback is now done entirely by
examination of the content listed in the response from the GData API.