PlaybackNext replaces the current playback object with new media.
Optionally, it waits until the end of the media. In this case,
it only waits until the end of the current loop.
EndLoop stops the playback at the end of the current loop.
Note that PlaybackNext and EndLoop have no effect if there is
no media currently playing.
This means that less confusion will reign if a button is pressed
at the wrong time, and catch-up needs to happen. Randomly switching
out lyrics/images for things scheduled from the previous tranche
just leads to chaos.
