Save As - track stop buttons in Activity instance

Track the stop buttons for the activity, so that they can be set
insensitive or not in a later patch.

Our API allows for more than one stop button, though the use of more
than one is very rare.

src/sugar3/activity/activity.py

@@ -448,6 +448,10 @@ class Activity(Window, Gtk.Container):
         bundle = get_bundle_instance(get_bundle_path())
         self.set_icon_from_file(bundle.get_icon())
 
+        self._stop_buttons = []
+
+    def add_stop_button(self, button):
+        self._stop_buttons.append(button)
 
     def run_main_loop(self):
         Gtk.main()

src/sugar3/activity/widgets.py

@@ -86,6 +86,7 @@ class StopButton(ToolButton):
         self.props.tooltip = _('Stop')
         self.props.accelerator = '<Ctrl>Q'
         self.connect('clicked', self.__stop_button_clicked_cb, activity)
+        activity.add_stop_button(self)
 
     def __stop_button_clicked_cb(self, button, activity):
         activity.close()