I've just realised that the JS puzzles' permalinks were not updating
when the user pressed 'n' for a new game, because all the front end
knows is that it passed a keystroke to the puzzle, and it has no way
of hearing back that a particular keypress resulted in a game id
change.

To fix this, I've renamed midend_request_desc_changes to
midend_request_id_changes and expanded its remit to cover _any_ change
to the game ids. So now that callback in the Emscripten front end is
the only place from which update_permalinks is called (apart from
initialising them at setup time), and that should handle everything.

[originally from svn r9805]