Extend download functionality in Favorite Broadcasts tab

Right now we have support for downloading all non-downloaded favorite broadcasts at once.

Ideally, you could launch downloads for individual items and delete individually. So, the Broadcast Favorites tab would also extend into a general purpose download manager. This idea would also necessitate some abstraction for downloading as mentioned in #43.

Each broadcast item must have an icon which reacts to an observation of a completed download in the downloads folder, similar to how the one in BroadcastDetailsFrag works. I attempted this briefly but since we're dealing with items in an adapter inside a ViewPager, this implementation is much more complex.