Rework AEItemStack (#3091)

* Use itemstack as itemdef
* HIGH_TAG/LOW_TAG should be compared both directions
* Remove getTagCompound
* Make Itemlist implementation independent
* Cache item id for performance reasons
* Add preconditions to saveguard against external meddling
* Chache itemDamage
* Remove IAEStackSearchKey for now, rename getDisplayStack

src/main/java/appeng/client/me/InternalSlotME.java

@@ -42,7 +42,7 @@ public class InternalSlotME
 
 	ItemStack getStack()
 	{
-		return this.repo.getItem( this.offset );
+		return this.getAEStack() == null ? ItemStack.EMPTY : this.getAEStack().asItemStackRepresentation();
 	}
 
 	IAEItemStack getAEStack()