Fixed #2170: sound applet player status icons

There are two variants of solving #2170:

- f0c53fb: the cinnamon theme designer edits the icons `player-playing`, `player-paused` and `player-stopped` and these are used; in older version this was hardcoded
- 7ca1101: The easier idea by @JosephMcc uses the default icons `media-playback-start`, `media-playback-pause` and `media-playback-stop`