Implement implicit instancing of editable objects

[AndrewPrifer]

For now, instancing is possible through having multiple editable objects in the scene with the same uniqueName. These Objects will share all editable properties. Since this is a dangerous thing to do (you won't get any warning if you unintentionally have duplicate uniqueNames), you'll have to opt into this by setting the allowImplicitInstancing prop of EditableManager to true, otherwise you get an error.

If you do opt into this behavior, only one instance will be visible in the editor.

This behavior is temporary until prefabs are implemented, and is discouraged, since you also opt out of any warnings for unintentional uniqueName duplicates. See #33.