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/target.js

@@ -37,4 +37,12 @@ Target.prototype.getName = function () {
     return this.id;
 };
 
+/**
+ * Call to destroy a target.
+ * @abstract
+ */
+Target.prototype.dispose = function () {
+
+};
+
 module.exports = Target;