The GUI uses the Gtk+ library.  Many parts are defined using
GtkBuilder files, which have a '.ui' prefix.  These are XML files and
as such can be editied using a text editor, which is often easiest
when making small changes.  More substantial changes however are best
done using Glade.  Since however PSPP has many custom defined widgets,
Glade must be started with certain environment variables set.  The
easiest way to run Glade with the correct variables is as follows:

    make src/ui/gui/glade-wrapper
    ./src/ui/gui/glade-wrapper <name-of-gtk-builder-file>

If you create new custom widgets for use in PSPP you must do the following to
make sure they are available to Glade:

* Add a call to its '_get_type' function to preregister_widgets(),
  which is found in widgets.c.

* Add the name of the module to
  src_ui_gui_libwidgets_essential_la_SOURCES in automake.mk.