In gWidgetsRGtk2 there is an idle handler that updates the top-level
components. However, changes are made below this, they will not be
updated automatically. Rather, the user must close and expand that
level.
The handler is called on the widget that provides the display, and not
the gvarbrowser widget. If you want that in the handler, be sure to
pass it in via the action argument.
svalue returns a character string containing the selected variable.