Add ability to add songs to the play queue

[jgoguen]

It would be nice to have the ability to add songs to the play queue, both as "Play Next" (adds to the front of the queue) and "Add to Queue" (adds to the end of the queue). If no song is currently playing this should create a play queue of one song, which starts playing when added to the queue. If other songs are playing, regardless of how playback was started, this should add the song to the appropriate position within the queue.