Web activity: quit the GTK main loop when activity is destroyed

Fixes #4603 .

In GTK activities, this is done by _ActivitySession when the activity
unregisters itself, in _complete_close method.

Making the web activity aware of session logout will be handled in
another patch for the shell, see http://bugs.sugarlabs.org/ticket/4611

src/sugar3/activity/webactivity.py

@@ -44,6 +44,7 @@ class WebActivity(Gtk.Window):
 
         self.connect("key-press-event", self._key_press_event_cb)
         self.connect('realize', self._realize_cb)
+        self.connect('destroy', self._destroy_cb)
 
         context = WebKit2.WebContext.get_default()
         context.register_uri_scheme("activity", self._app_scheme_cb, None)
@@ -69,6 +70,10 @@ class WebActivity(Gtk.Window):
         SugarExt.wm_set_bundle_id(xid, self._bundle_id)
         SugarExt.wm_set_activity_id(xid, str(self._activity_id))
 
+    def _destroy_cb(self, window):
+        self.destroy()
+        Gtk.main_quit()
+
     def _loading_changed_cb(self, web_view, load_event):
         if load_event == WebKit2.LoadEvent.FINISHED:
             key = os.environ["SUGAR_APISOCKET_KEY"]