Entity re-parenting

It's helpful to be able to entities to different parents. Currently, to do this, you need to:

1. flush entity to DOM (to preserve your entity state).
2. copy the entity.
3. add copy to new parent entity element.
4. remove the existing entity.

var entity = document.querySelector('#myelement');
var newParent = document.querySelector('#mynewparent');
entity.flushToDom();
var copy = entity.cloneNode();
newParent.appendChild(copy);
entity.parentNode.removeChild(entity);

To do this in three.js, it's a simple matter of changing the object3d.parent. DOM parentNode and parentElement are read only properties, so you cannot re-write to a new parent. Not sure if this applies any differently to custom element/web component.

It would be nice to just:

entity.parent = newParent;

Also welcome any suggestions for any alternative way of doing this that I may have missed.

Start by investigating the entity, parent, flushToDom, cloneNode, appendChild, and parentNode behavior described in the issue, including how custom elements or web components handle re-parenting. Done should mean an entity can move to a new parent without flushing, copying, and removing it while preserving its state.