core: JsonParser: add support for alternative qrc scheme

Also fixes a minor reference error.
core: GCAudio: Expose playbackState

This mostly proxies through Audio.playbackState. We also report
errors as Audio.StoppedState because Audio's playbackState is
PlayingState also in case of error.

Although this is enough to serve our current needs in future we might
need or want to expose a more complete XxxState property in GCAudio.
click_on_letter: Delay audio if intro audio is running