It looked identical in both states, so the only way to tell what
pressing it would do was to look at the other side of the window and
infer it -- and for anyone not looking there was nothing to infer from:
no aria-expanded, no aria-controls, no drawn state.
The state is reflected *from the panel* rather than kept beside the
click. This button is not the only thing that opens the queue --
now-playing-view sets the same attribute, because it hides the bar the
button lives in -- so a flag maintained by the click handler would be
right until something else opened the panel and then quietly wrong.
The panel's `open` attribute stays the one fact; a MutationObserver
reflects it.
Refs #26
