youtube: Add a GDataYouTubeFeed helper class for parsing feeds

This parses YouTube-specific properties of the feed — specifically, the
total-results and items-per-page, which were previously handled in a
service-agnostic manner by GDataFeed, but YouTube doesn’t conform to
that.

This adds the following new API:
 • GDataYouTubeFeed

https://bugzilla.gnome.org/show_bug.cgi?id=687597