Move Metadata Discoverer to AudioObject?

[danirabbit]

Currently the metadata discoverer runs when adding files to the queue, which is fine for this design, but if there are future views, we probably want metadata even for audio objects that aren't queued.

Now that metadata discovery isn't attached to the playbin bus, we can probably move this to AudioObject so that it runs whenever an audio object is created. This would make all metadata storage pretty self contained there and keep the playback manager scoped to only handling playback and not also metadata