allow pause while playing

There are situations when CanPlay is false while CanPause is true. Xmms2
for example permits deleting the song currently playing without stopping
it. So CanPlay shall only affect the button while no song is active,
expressed through the state STOP.
always refresh status before display

While the button is disabled, its functionality can still be triggered
via (Media) Hotkeys. So before taking any means induced by a changed
status, check whether the status really changed.