gui: Fix Glib warnings for dialogs in realize and configure callbacks.

The GtkBuilder documentation says:

    Prior to 2.20, GtkBuilder was setting the "name" property of
    constructed widgets to the "id" attribute. In GTK+ 2.20 or newer,
    you have to use gtk_buildable_get_name() instead of
    gtk_widget_get_name() to obtain the "id", or set the "name"
    property in your UI definition.

This commit fixes the problem by switching from using the "name"
property to calling gtk_buildable_get_name().