the defaults are to keep existing behavior, and display all fields.
new boolean options are added for:
- show-previous shows "<<" icon to rewind the player
- show-play-pause shows "||" or "|>" icon to play/pause the player
- show-next shows ">>" icon to advance the player
- show-name shows the media name (e.g. the artist + title) and
the number of players
these are especially useful for narrow screens, like mobile phones,
where a user may wish to disable all but "show-play-pause" in order to
fit size limitations of the panel.