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

@param onShown action to be performed
@return builder for chaining
workbenchfx-core/src/main/java/com/dlsc/workbenchfx/model/WorkbenchDialog.java