Added new block prefabs

Put the 2D Physics Materials used for blocks in a new folder to keep the
names simpler. Oh yeah, renamed those materials. Added a new 'Slippery'
material.

Added a new prefab for Bouncy blocks, it's red.

Added a new prefab for Slippery blocks, they're blue.

Replaced the big block with one using the bouncy prefab, replaced one
platform with slippery blocks.