Implement Chu Shogi

Implement Chu Shogi

Chu Shogi is added as a new variant, usig the extended piece set.
Implement Lion double-moves. Apart from a from- and a to-square, each move
now also has a kill square (all represented by X and Y coordinate). This
is set on receiving a pair of partial moves (and passed around as a global;
sorry about that). MakeMove uses the kill-square to remove an extra piece,
and append it as ";SQUARE" to the long-algebraic move. SendMoveToProgram()
converts this back to a double move of the Alien protocol (two comma-separated
long-algebraic moves, stepping via SQUARE).
  For user input, releasing or to-clicking on a cyan square sets the kill-
square, and sends a lift command to prompt the engine for the markers for
the second leg. The to-click of this second leg then determines the to-square.