"Reaction to pistons" option for blocks

This is (yet) another property determined by the material, but it's possible to override it with this piece of code:

public EnumPushReaction getMobilityFlag(IBlockState state) {
	return EnumPushReaction.REACTION;
}

The possible reactions are:

• NORMAL,
• DESTROY: pistons break the block when extending (e.g. replaceable plants, pumpkins)
• BLOCK: pistons won't extend (e.g. obsidian, bedrock)
• PUSH_ONLY: the block won't be pulled back by sticky pistons (e.g. glazed terracotta)
• IGNORE: not really sure, seems to behave like PUSH_ONLY