gtkwidget: Avoid poking surfaces and events in gtk_widget_list_devices()

gtkwidget: Avoid poking surfaces and events in gtk_widget_list_devices()

This got stuck in ancient times when widgets were windowed, so the devices
in a window to know the devices in that widget would pan out. We do only
want here the devices that are inside the widget, not spread over the
surface, so rewrite this helper function to poke the toplevel foci, and
look they are contained inside the widget.