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.
The trouble here is that this only makes the variables available within the body part of the modal. Note that `wiki.makeTranscludeWidget` takes an optional parameter `options.variables` that is a hashmap of variable values to set within the transclusion.

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).