Limit undo depth to 50.  This should save a bit on the size of saved projects.
Limit undo depth to 50.  This should save a bit on the size of saved
projects.

Related to #617.

google/codeworld,alphalambda/codeworld,alphalambda/codeworld,google/codeworld,pranjaltale16/codeworld,google/codeworld,pranjaltale16/codeworld,pranjaltale16/codeworld,alphalambda/codeworld