Allow use of second-row pieces for non-promoted in drop games

Allow use of second-row pieces for non-promoted in drop games

Second-row pieces were automatically demoted on capture, but this
should really happen only when the piece-to-char table says it is a
shogi-promoted piece (assigned a '+' there). Pieces assigned a letter
should really stay themselves.
 PieceToNumber had to be adapted to skip '+' pieces in order to count
correctly for second-row pieces, as did the code in UserMoveEvent() that
mapped the from-square onto the piece type. (Why do we do that anyway?
The piece type was displayed on the from-square!)