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: scene.lookup("#myId");.

Returns:  the id of this Styleable