Fix music player's sound management

If an operating music player was disconnected from the electrical network
or destroyed, it used to leave the music playing forever.  There was
also a glitch upon starting playing, as the music was started by the
form handler but then stopped when the run function realised it wasn't
receiving any power, because it hadn't demanded any yet.
Simplify music player interface

Having a track selected now implies playing it, rather than
playing/stopped being a hidden state separate from track selection.
Option for forcefield to be cubical
Fix forcefield power demand when disabled