Giving users of modal a chance to reference the underlying tiddler

To make it possible to reference the underlying tiddler from a modal the `currentTiddler` variable is set and made accessible in the `bodyWidgetNode`.

This way the user (programmer) gets a chance to access fields of the tiddler the modal is based on.
> I think we should also default currentTiddler to the title of the modal tiddler (but users would be able to override its value if they wanted).

You mean default currentTiddler to the subtitle which is used as title?