#125 Last move centered in PGN window

[Roy E]

When stepping through a game on the board, it would be nice if the PGN window would scroll not just when the last move gets to the very bottom of its window, but when the last move is in the center (vertically). That way the user can see the surrounding moves and annotations, both above (before) and below (after) that move.

tl;dr: Keeping the current/last move centered in the PGN window would be very useful.

(Using 5.0.2 in Windows.)