Support TinyTinyRSS/OldReader-like services

[JadedCtrl]

Doesn't seem like this'll require too much restructuring― the FeedsList should become a BOutlineListView (which has, e.g., "TinyTinyRSS" and "Local" as the top of the hierarchy, with their feeds under them), and new Feed classes should be made. Other than that, a class above Feeds ("MetaFeed", "SuperFeed"? Haha) could be used to fetch subscribed feeds. It might represent the service/way feeds are aqquired (i.e., LocalMetaFeed, TTRMetaFeed, etc).

Right now, subscribed feeds are just fetched from the FeedController― so instead of using FeedController::SubscribedFeeds, you'd use MetaFeed::SubscribedFeeds, and FeedController might have to manage multiple MetaFeeds.

Using the addon system for MetaFeeds and Feeds might be a good idea, to help generalize.