Fix resource leaks discovered while testing

- `Drawable` now removes its `Skin` on `dispose`, disconnecting events.
- Creating a new `Drawable` doesn't always create a new `Skin` any more:
  now it can share an existing skin.
- Don't throw when asked to update the properties of a `Drawable` that
  has already been destroyed. This seems like a VM bug but was causing
  overwhelming output in the browser for some projects.

src/Drawable.js

@@ -69,9 +69,8 @@ class Drawable {
      * Dispose of this Drawable. Do not use it after calling this method.
      */
     dispose () {
-        if (this._id >= 0) {
-            delete Drawable[this._id];
-        }
+        // Use the setter: disconnect events
+        this.skin = null;
     }
 
     /**