Rhythmbox plugin: Bug fix, update for modern MonoDevelop and new features

As discussed via e-mail with Chris Halse Rogers.

Taken together, these commits:
* Adds a .gitignore (this is the first commit made to this repo after conversion from LaunchPad/Bazaar).
* Updates the Rhythmbox project to one usable in modern MonoDevelop versions, and changes the plugin homepage (used by the About button in Do) to one that isn't taken over by spam.
* Fixes a bug in said plugin, which incorrectly grouped songs when multiple albums shared the same title, but were from different artists.
* Adds sorting by disc number, not just track number; however, sorting is mostly ignored in Do at the moment (it sorts results by relevance, regardless of the order we provide), so this might not be very useful yet.
* Adds all playlists and songs to the search index. For playlists, the Rhythmbox MPRIS plugin must be activated (which it appears to be by default), as rhythmbox-client doesn't support switching playlists. Everything else keeps working as before if the plugin happens to be disabled.
* Uses Do.Platform.Log instead of Console.WriteLine and Console.Error.WriteLine for error logging.

I did not add myself to the COPYRIGHT file, by the way; Chris is also not listed. Not sure if this should be updated.
Do should (unless I've broken it recently ☺) refresh once a minute on AC and once every 5 minutes on battery. We also try to keep Universe updates to <100ms, so if you've got a lot of plugins enabled (or a couple of slow plugins hitting the network) you might not get refreshed each time.

Regardless, I'm happy for DynamicItemSource to be a bonus round feature :+1: