Remove promotion-piece encoding from ChessMove type

Remove promotion-piece encoding from ChessMove type

The shizophrenic way to indcate promotion pieces in the XBoard code
(sometimes using a promoChar, at other times relying on moveType) was
cured by combining all promotion move types (such as WhitePromotionRook)
into a single one (WhitePromotion), and relying on promoChar to specify
the piece. This solves the problem of promoting to new piece types in
variants, which formerly also required addition of new move types.

In Shogi the '+' character is now consistently used to indicate
promotions (and recognized in ApplyMove), rather than 'q' or
PieeToChar(BlackQueen), as the latter did not work in minivariants where
the Queen (representng the Lance) did not partcipate.