ItemDef/EntityDef/Block: Add method cloneTo to definition to other definition

ContentBuilder: Add method `get` to get definition or nullptr
ContentLoader: Add functionality to clone from definition specified in `parent` field in json

src/items/ItemDef.cpp

@@ -5,3 +5,13 @@
 ItemDef::ItemDef(const std::string& name) : name(name) {
     caption = util::id_to_caption(name);
 }
+void ItemDef::cloneTo(ItemDef& dst) {
+    dst.caption = caption;
+    dst.stackSize = stackSize;
+    dst.generated = generated;
+    std::copy(&emission[0], &emission[3], dst.emission);
+    dst.iconType = iconType;
+    dst.icon = icon;
+    dst.placingBlock = placingBlock;
+    dst.scriptName = scriptName;
+}