basic "bubble" mechanic, and new enemy type

Began work on the bubble mechanic. py_objectSettings now stores if the
enemy is currently in a bubble, and the specific enemy control reads
this to determine how it behaves. At the moment it only effects the
spider, which will turn upside down and freeze for 300 frames.

Also included an enemy type that runs away, dropping salt when player
gets close. Ultimately this enemy will run around at random, and salt
will damage the player.