Grassify replaces podzol

[Anna Bakker]

The /grassify command replaces podzol and dirt that can not naturally be turned to grass (which is coarse dirt in 1.8 and spawns in mesa and mega spruce biomes).

These two blocks have the same id as normal dirt, but different damage values.

[Anna Bakker]

don't replace podzol and coarse dirt, resolves #401

→ <<cset 7e461e45adb2>>