The ::map-event signal will be emitted when the widget's window is
mapped. A window is mapped when it becomes visible on the screen.
To receive this signal, the gdk.Window associated to the widget needs
to enable the GDK_STRUCTURE_MASK mask. GDK will enable this mask
automatically for all new windows.
TRUE to stop other handlers from being invoked for the event.
FALSE to propagate the event further.