Add option -jewelled to decide which King is a Zebra XB

Add option -jewelled to decide which King is a Zebra XB

A new volatile option -jewelled is added, with default value -1.
When it is set to 0 the King image will be replaced by the Zebra image,
when it is 1 the same thing will happen to the black King. This color
testing will be done before the swapping of colors ordered by -flipBlack,
so that the color rather than the side of the (possibly flipped) board
decides which King is displayed as Zebra. (While the board side determines
whether a white or black Zebra will then be used in -flibBlack mode).
Shogi themes can use this by providing a Jewelled General as Zebra (a piece
that was not used in any Shogi variant).