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.
Hi @felixhayashi I meant that the currentTiddler variable should be initialised with the title of the modal tiddler. It could still be overridden with a different value.