Added an init(...) function to the player interface.

All players must now provide both init() and
play() functions. Some of the arguments and
work have been moved from play() to init().
init() is called when the Start button is clicked.
play() is called when the Go button is clicked.
Complete and tested for ScorePlayer in this
commit.
Moved sequence performance code to prototype.

Since assisted performances use subsequences,
it must be possible to have many sequences, so
it was a mistake to try to use a namespace for the
sequence class. The code in ScorePlayer.js has
therefore been moved back into sequence.prototype.
Tested and running for non-assisted performances.
Localised the track initialisation functions.

Moved them from Sequence.js inside
Controls.sendTrackInitializationMessages().