Extend makeTranscludeWidget

Add support for specifying the filename, and the fallback content
within the transclude widget that is used if the transclusion target
isn’t found
Fix problem with refreshing modal dialogues

Previously any refreshing of the content of a modal would cause a
crash. The problem is the way that we steal the root widget for the
render trees used in the modal. The root widget is tied to the
container DOM node for the main content area, which isn’t actually a
parent of the modal DOM nodes, hence the confusion for the refresh
mechanism.