[Dialog] Dialogs with owner window set should inherit stylesheets

Using a Dialog<T> class to create popup dialogs. Under the covers, Dialog creates a HeavyweightDialog which creates a Stage and Scene. If the Dialog.initOwner() is used, it would make sense for the Scene created to copy in the stylesheets from the Scene of the owner Window so that the scene graphs share the same styles.

The workaround for me was to set a handler for Dialog.onShowing and inject styles on the Dialog.dialogPane.