Session#process_events_on should be able to listen for events on other objects

[Burgestrand]

Possible solutions:

1. Pass a Hash<Object,Event> (would allow one to listen for events on multiple objects):

   session.process_events_on(playlist_container => [:container_loaded], playlist => [:playlist_state_changed])

   Would be useful waiting for multiple tracks to load, for example (?). But… YAGNI?

2. Allow passing the object to listen to as the first parameter:

   session.process_events_on(playlist, :playlist_state_changed, :playlist_metadata_updated)

   This would only allow you to listen for events on a single object, and this is the simplest thing
   that could possibly work. I think I will go with this implementation first, and later refactor if needed.

Number 2 has the benefit of making it easier to implement #24, as well.

[Burgestrand]

Might be related to #56.

[Burgestrand]

Screw it. This issue’s been around for ages and appears to not have been needed. Most of the loading is done with polling (there’s a place to fix the API). At best case, #26 is pretty much the solution to this one as well.