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.
Our goal is to stick to consistent coding style across the TiddlyWiki code base, so I've some fairly low level feedback about layout and style. Here the issue is that there shouldn't be a space after the comma in a parameter list.