Show current, elapsed and remaining time on green background
Refactor seek function into audio_seek()

We'll need seeking multiple times, so refactor it to comply with DRY.
New stop and end-of-stream functionality

When stop is pressed onced or the end of a stream is reached, the player
rewinds the file to the start or loads the next file if already
selected.

Pressing stop again really stops the player.
[cosmetics] Rename socket_handler to joystick_handler