changes _updatePreviewBoard to only use non-null preferences and calls it on startup in ProfilePage
renames all instances of preference or preferences to 'prefs' abbreviation in User
renames Merida pieceset to Classic
adds DEFAULT_STYLE constant to Piece widget
adds better torn effect to preview board overlay and adds psd version to ProfilePage
adds DEFAULT_SQUARE_SIZE constant ( = Square.DEFAULT_SIZE) to Board widget
finishes prefs page - adds separate handlers to deal with different data types
and adds handler for HasIdentity to initialise the page.