atoms.lisp: Fix undo/redo action sensitivity.

Undo should be available after the first turn, but wasn't.  The reason
is twofold:

  * `update-undo-redo-sensitivity' was hooked onto :processing-move
    rather than :start-turn, which was just a mistake, though it seemed
    not to be because the order of the notifications was wrong; and

  * `perform-explosions', which is where :start-turn is issued, was
    being called before the undo record was laid down, whereas it should
    always have been afterwards.