Add renpy.music.is_music().

An internal function that returns true if a channel is a music
channel, and false otherwise.
Make Play's participation in get_selected optional.

It doesn't make sense for Play on the sound channel to participate
in get_selected(), since it's very likely the sound will quickly
end.

See: http://lemmasoft.renai.us/forums/ucp.php?i=pm&mode=view&f=0&p=148211
where Play causes Ren'Py to restart the interaction repeatedly.