tetris.el: Use `define-derived-mode'; fix window selection; doc fixes.

tetris.el: Use `define-derived-mode'; fix window selection; doc fixes.

* play/tetris.el (tetris, tetris-update-speed-function)
  (tetris-tty-colors, tetris-x-colors, tetris-move-bottom)
  (tetris-move-left, tetris-move-right, tetris-rotate-prev)
  (tetris-rotate-next, tetris-end-game, tetris-start-game)
  (tetris-pause-game): Fix typos in docstrings.
  (tetris-mode-map, tetris-null-map): Move initialization into declaration.
  (tetris-mode): Define with `define-derived-mode';
  set show-trailing-whitespace to nil.
  (tetris): Prefer window already displaying the "*Tetris*" buffer.