[finishes #70375260] changeScore block

Replaced the old incrementScore block that you let you add to the player or the opponent's score.

The new changeScore block is just "score point" for K1 levels and "score/remove point" for all other levels. Scores can go up or down from zero. The score doesn't appear until the first time the block is used (this can be overridden per-level by setting level.showZeroScore).

Note that the /2014/18 toolbox level uses a different setScoreText block where the student has the flexibility to place any arbitrary string in the score text.