[left mouse]: fill cell (draw)
    [middle/right mouse]: clear cell (erase)
    [up/down/left/right]: shift world (regenerates!)
                [delete]: clear state
                     [r]: (re)generate
                 [space]: pause/unpause
                     [n]: one step
                     [s]: save state
           [s] + [shift]: load state
                     [h]: hide/show gui