Makes an element an overlay with an optional backdrop.
iron-overlay-behavior displays an element on top of other content. It starts out hidden and is
displayed by calling open() or setting the opened property to true. It may be closed by
calling close() or cancel(), or by setting the opened property to true.
The difference between close() and cancel() is user intent. close() generally implies that
the user acknowledged the content of the overlay. By default, it will cancel whenever the user taps
outside it or presses the escape key. This behavior can be turned off via the no-cancel-on-esc-key
and the no-cancel-on-outside-click properties.