The id of this
Styleable. This simple string identifier is useful for
finding a specific Node within the scene graph. While the id of a Node
should be unique within the scene graph, this uniqueness is not enforced.
This is analogous to the "id" attribute on an HTML element
(CSS ID Specification).
For example, if a Node is given the id of "myId", then the lookup method can
be used to find this node as follows: