Event listeners are not being removed when an entity is destroyed

Adding entities (or any kind of Cesium object with a definitionChanged object) to an Entity as a non-standard key causes a listener to be added to it, and not removed.

Here's a Sandcastle example showing the issue: https://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=f0bbec7cd261365aaadd3bb991c4261c

Reported on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/bRk7kmcApeo

Start by reproducing the issue in the linked Sandcastle example, then trace how non-standard Entity properties with a definitionChanged object register listeners. Done means the listener is removed when the Entity or contained object is destroyed; the linked forum report provides additional context.

Written by the indexing model from the issue text.