Gertty uses WidgetWraps around the LineBox widget for popup widgets. Add
LineBoxTitlePropertyMixin which adds property and setter for title attribute,
accessing the underlying title_widget on the wrapped LineBox widget.
We use the popup method in the controller to display these dialogs. It uses an
Overlay widget to display our dialog widget on top of the current view. If the
widget to be displayed by the Overlay widget has title attribute, copy it to
the Overlay widget.
Change-Id: I4c6ecbc7ed87867cd67cd93c1125384dd5d4b9af