Stage, costumes, backdrops (#149)

* Add `Clone.prototype.getCostumeIndexByName`, keep in range

* Add basic costume primitives from Scratch 2.0

* Add costume getter block

* Add properties and methods for distinguishing stage and sprites-vs-clones

* Add backdrop-related looks blocks

* Fix up "switch to backdrop" to be working

* Costume/backdrop reporters are 1-indexed

* Fire "when backdrop switched" hats

* Cut cloning helpers for a separate PR

* Disable many blocks on the stage

* Refactor into _setCostumeOrBackdrop; implement switch backdrop and wait

* Fire hats even when backdrop unchanged

src/engine/runtime.js

@@ -434,6 +434,19 @@ Runtime.prototype.getTargetById = function (targetId) {
     }
 };
 
+/**
+ * Get a target representing the Scratch stage, if one exists.
+ * @return {?Target} The target, if found.
+ */
+Runtime.prototype.getTargetForStage = function () {
+    for (var i = 0; i < this.targets.length; i++) {
+        var target = this.targets[i];
+        if (target.isStage) {
+            return target;
+        }
+    }
+};
+
 /**
  * Handle an animation frame from the main thread.
  */