* basep_widget.c  foobar-widget.c menu.c
Use GtkButton's in_button field instead of ButtonWidget's

* button-widget.[ch]:
Derive ButtonWidget from GtkButton instead of GtkWidget; use
GtkButton's pressed, released and clicked signals instead of signals
defined in ButtonWidget; ButtonWidget now can receive focus and react
to keystrokes.
Remove calls to g_return_if_fail (foo != NULL) if followed by calls
to g_return_if_fail (GTK_IS_BAR (foo));