Add items in backdrop menu

src/lib/blocks.js

@@ -44,7 +44,9 @@ export default function (vm) {
 
     const backdropsMenu = function () {
         if (vm.runtime.targets[0] && vm.runtime.targets[0].sprite.costumes.length > 0) {
-            return vm.runtime.targets[0].sprite.costumes.map(costume => [costume.name, costume.name]);
+            var a = vm.runtime.targets[0].sprite.costumes.map(costume => [costume.name, costume.name]);
+            a.push(["next backdrop", "next backdrop"], ["previous backdrop", "previous backdrop"]);
+            return a;
         }
         return [['', '']];
     };