Suggestion: add playback control buttons

Add a **repeat** button which toggles between:
 - repeat current module (`repeat 1`)
 - play next track (`repeat`)
 - stops at the end of the current track

Default option could be the second: `repeat`.

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Add `next`/`prev` buttons:

 - plays next/previous track (doesn't change playback status)
 - cycles through the playlist if first/last item are reached