Liquids: Flow into and destroy 'floodable' nodes

Add new node property 'floodable', default false
Define "air" as floodable = true in C++ and lua

builtin/game/item.lua

@@ -574,6 +574,7 @@ core.nodedef_default = {
 	diggable = true,
 	climbable = false,
 	buildable_to = false,
+	floodable = false,
 	liquidtype = "none",
 	liquid_alternative_flowing = "",
 	liquid_alternative_source = "",