Event handler for leaving the file name input field.
If the input value did nit change, hide the file name input field and show
the file name text again. If the value in the input field has changed, keep the input
field and mark it as changed.
Construct the UI for this widget from a template, setting this.domNode.
Lookup cached version of template, and download to cache if it
isn't there already. Returns either a DomNode or a string, depending on
whether or not the template contains ${foo} replacement parameters.