The "map" signal does not mean a widget is mapped. It may be emitted
before. Yet pointer and keyboard grabs won't work on non viewable
widgets and will fail with GDK_GRAB_NOT_VIEWABLE.
For some reason, a GtkWindow with type GTK_WINDOW_POPUP was always
mapped when "map" is emitted but not yet with type GTK_WINDOW_TOPLEVEL
(at least in my tests). Thus switch to "map-event" to make sure the
widget is actually mapped.
