This patch adds an option to the menubar for exporting the current
game as a PGN file. This file can then be read by other chess
programs (and ours eventually) to replay the game or analyze it.
The implementation is mostly PGN spec compliant, however the code
could use some more work. Particularly the `const_cast`s...
But it's a start. :^)
Fixup: Chess: Fixed hard-coded home path in unveil() call
Fixup: Chess: Removed castling flags from Move struct
The castling detection logic is done inside Move::to_algebraic()
now, removing the need for is_castle_short and is_castle_long flags
inside of the Move struct.
As much as I like the "test" piece set, I think "stelar7" is a much
better first impression for anyone opening the Chess game, so I
consider it a much more sensible default.
