Add the ability to store user data in the Element

[PavelTurk]

Sometimes there is a need to store user-defined data in an Element. Since Element forms a tree structure, creating a wrapper around it is not a viable solution, because it is impossible to obtain a reference to the wrapper from a given Element (for example, element.getParent().getWrapper()). Therefore, I propose adding a simple properties map, similar to the one in JavaFX’s Node.