playlist URLs

A playlist URL opens its page in the Browser so you see all its videos first (handpick if you want). It does not auto-import. From there you can do either or both:

• Save it in the Browser left bar, the way you keep channels (the playlist becomes a first-class browser citizen you can revisit).
• Convert it into a lore playlist, pulling it into your library as a collection (videos arrive not-downloaded; download what you want).

Either way it remembers the source URL so it can re-sync. Channels are the same shape (see #27).

Spans several layers: URL routing, playlist-as-browser-citizen, source-linked re-sync. Not a small change.

Raised on Reddit: https://www.reddit.com/r/macapps/comments/1u78np7/comment/otur345/ (reply to the commenter when this ships).