Play/pause and skip to previous/next song work. The currently playing song
name is shown on the Cor. The track length and progress are now shown as
we don't know how to send these yet.
Add actions to the filter (this should help with #536)
Add "copy" constructors to MusicSpec and MusicStateSpec, and use those when receiving an updated intent, this way partial updates do not disrupt the local information.
Iterate over incoming extra keys, explicitly check the incoming type and use only known type. This could help with #533
Possible problem: this code iterates over every key of the incoming bundle.
