Clones (#150)

* Provide property to Clone to distinguish "original" clones

* Provide method to clone a clone's properties

* Don't report clones in the UI target list

* Add target info to Thread

* Allow hats to skip clones (for green flag)

* Green flag skips clones

* Implement "create clone" and hat

* Pass the runtime to sprites and clones (for start hats)

* Clone disposal; trigger hats after drawable initializes.

* Separate stop threads for target; fix handling of stop button

* Remove extraneous `skipClones` property

* Add global clone limit

* Don't allow a non-clone to delete itself.

* Rename `cloneClone` -> `makeClone`

* Variable updates in runtime.js

* Synchronous drawable initialization (until we put it back to promises)

src/engine/thread.js

@@ -29,6 +29,12 @@ function Thread (firstBlock) {
      */
     this.status = 0; /* Thread.STATUS_RUNNING */
 
+    /**
+     * Target of this thread.
+     * @type {?Target}
+     */
+    this.target = null;
+
     /**
      * Whether the thread requests its script to glow during this frame.
      * @type {boolean}
@@ -145,4 +151,20 @@ Thread.prototype.setStatus = function (status) {
     this.status = status;
 };
 
+/**
+ * Set thread target.
+ * @param {?Target} target Target for this thread.
+ */
+Thread.prototype.setTarget = function (target) {
+    this.target = target;
+};
+
+/**
+ * Get thread target.
+ * @return {?Target} Target for this thread, if available.
+ */
+Thread.prototype.getTarget = function () {
+    return this.target;
+};
+
 module.exports = Thread;