Revert "ItemBlock: drop the charade about overriding built-in block types"

This reverts commit f64dc01bd1.

I forgot that the ItemBlock constructor implicitly strips off any states
of the origin block, which is something that we unfortunately can't do
any other way right now, since the blocks don't remember their default
states.

src/block/UnknownBlock.php

@@ -38,7 +38,7 @@ class UnknownBlock extends Transparent{
 		$this->stateData = $stateData;
 	}
 
-	public function describeType(RuntimeDataDescriber $w) : void{
+	protected function describeType(RuntimeDataDescriber $w) : void{
 		//use type instead of state, so we don't lose any information like colour
 		//this might be an improperly registered plugin block
 		$w->int(Block::INTERNAL_STATE_DATA_BITS, $this->stateData);