public final ObjectProperty<EventHandler<Event>> onShownProperty() {
    return onShown;
  }
The dialog's action, which is invoked whenever the dialog has been fully initialized and is
being shown. Whenever the {@link #dialogProperty()}, {@link WorkbenchDialog#buttonTypes},
{@link #buttonTextUppercaseProperty()} or {@link #workbenchProperty()} changes, the dialog will
be rebuilt and upon completion, an event will be fired.

@return the property to represent the event, which is invoked whenever the dialog has been
    fully initialized and is being shown.
workbenchfx-core/src/main/java/com/dlsc/workbenchfx/view/controls/dialog/DialogControl.java