Report a Volume of 0.0 for a muted player and set its muted property to whether volume is 0.0

Unfortunately there's no Muted property on MPris, so you cannot easily "restore" the previous volume, but that's an issue that affects every player, so when you click mute on a player and then adjust the volume in media controller it raises the volume from 0 rather than just working on the previous volume like the volume plasmoid would

• Clicked mute on a video player, mpris reported zero, and UI reflected muted state by crossing out the volume icon on the player
• Used wheel on media controller to change volume, media player became muted when volume hit 0.0

Shouldn't have any compatibility side-effects since we assume muted to be false by default so if this field isn't present because mixing older host with newer extension, nothing would change in behavior here.