quick frontends (widgets): added widgetNew and widgetDelete listeners:

- `widgetNew` is called when a brand new widget is created (not when a new instance is
  created if there is already an existing one)
- `widgetDelete` is called when all instances of a widget (with a given widget_hash) have
  been deleted.

sat_frontends/quick_frontend/quick_app.py

@@ -560,8 +560,13 @@ class QuickApp(object):
                 args: (selected_widget,)
             - notification: called when a new notification is emited
                 args: (entity, notification_data, profile)
-            - notification_clear: called when notifications are cleared
+            - notificationsClear: called when notifications are cleared
                 args: (entity, type_, profile)
+            - widgetNew: a new QuickWidget has been created
+                args: (widget,)
+            - widgetDeleted: all instances of a widget with specific hash have been
+                deleted
+                args: (widget_deleted,)
             - menu: called when a menu item is added or removed
                 args: (type_, path, path_i18n, item) were values are:
                     type_: same as in [sat.core.sat_main.SAT.importMenu]