FileChooserButton signal how to?

[Damon Register <damonregister bellsouth net>]

I am just learning gtkmm using the user interface built with Glade3.  I
have found a few examples and with the reference document at
http://www.gtkmm.org/docs/gtkmm-2.4/docs/reference/html/classes.html
I have been able to learn many of the widgets that Glade3 has.  Most of
the widgets were quite easy to setup and use but I am stuck on trying
to figure out the FileChooserButton.

From the gtkmm doc I found get_filename() and was able to get the file
name after it was selected.  It seems logical (at least to me) that
there should be a signal to tell me after that the selection is complete
but I haven't found the method to do that.  The closest thing I could
find is the signal_update_preview() but I see that happens as soon as
I select a file and not when I click the open button.  How do I get the
signal from the open button in the chooser dialog?

Damon Register