(bubbles-version): Bump value to "0.5".

(bubbles-version): Bump value to "0.5".
(bubbles-mode-map): Move define-key statements here.
(bubbles-game-theme-menu): Ditto.
(bubbles-graphics-theme-menu): Ditto.
(bubbles-menu): Ditto.
(bubbles-mode): Initialize buffer-undo-list, redisplay.
(bubbles--initialize): Reset buffer-undo-list, redisplay.
(bubbles-plop): Set buffer-undo-list, redisplay.
(bubbles-undo): Reset buffer-undo-list, redisplay.
(bubbles--show-images): Take care of missing text properties.