applet: Add fallback icons in device view and stream view
For device view, use `audio-volume-muted` if there is no output or input devices. For stream view, use `edit-none` if there is no applications playing or recording audio.
For device view, use `audio-volume-muted` if there is no output or input devices. For stream view, use `edit-none` if there is no applications playing or recording audio.