Play Bubbles game.
Uses keymap `bubbles-mode-map', which is not currently defined.
The goal is to remove all bubbles with as few moves as possible.
M-x bubbles-plop on a bubble removes that bubble and all
connected bubbles of the same color. Unsupported bubbles fall
down, and columns that do not contain any bubbles suck the
columns on its right towards the left.
M-x bubbles-set-game-easy sets the difficulty to easy.
M-x bubbles-set-game-medium sets the difficulty to medium.
M-x bubbles-set-game-difficult sets the difficulty to difficult.
M-x bubbles-set-game-hard sets the difficulty to hard. (fn)