Refine block list for tornadoes

[UndeadZeratul]

It shouldn't pick up everything in its wake, that's just infuriating. Rather, anything that's "loose" should be at risk. Things like the following should all be at risk of being grabbed by tornadoes:

• wood planks
• sand
• gravel
• cobblestone (maybe)

This way on the rare chance that a tornado does fly by, the terrain isn't smashed to pieces, and everything the player has worked towards won't be in vain. Basically only shoddy beginner materials should be at risk.