add an "x" icon to hide players

Submitted by unh..@..@mm.st

Assigned to Andrzej

As noted at https://www.reddit.com/r/xfce/comments/9x51lo/how_to_hide_other_media_players_not_currently/ , it'd be nice to be able to remove players currently not playing from the panel volume plugin.

Maybe simply an "x" icon which, when clicked, removes them, like

✘ Amarok ⏪⏸⏩ Not currently playing ✘ Parole ⏪⏸⏩ Not currently playing