Add gravity puffers

[swoolcock]

Puffers that optionally change Madeline's gravity on explosion or bounce (configurable).

They should probably also have their own inversion state that is affected by gravity springs they touch. An inverted puffer would flip the sprite and all colliders.

[swoolcock]

Ideas:

• If Madeline triggers a bounce (regardless of gravity state), the puffer will move in the direction you push it.
• If Madeline triggers an explosion, her gravity will change to that of the puffer (no toggle).
• Puffers will have inverted sprites and colliders if their gravity type is "inverted".
• If a puffer is moved by a gravity spring it will change gravity to match the spring, then move in the direction the spring pushes it.
• If a puffer moves through a gravity field and "affects non-holdables" is enabled, the puffer will change gravity.