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_widgets.py

@@ -231,9 +231,10 @@ class QuickWidgetsManager(object):
 
         if widget is None:
             # we need to create a new widget
-            log.debug("Creating new widget for target {} {}".format(target, cls))
+            log.debug(f"Creating new widget for target {target} {cls}")
             widget = cls(*_args, **_kwargs)
             widgets_map.setdefault(hash_, []).append(widget)
+            self.host.callListeners("widgetNew", widget)
 
             if on_new_widget == C.WIDGET_NEW:
                 self.host.newWidget(widget)
@@ -341,6 +342,7 @@ class QuickWidgetsManager(object):
             del widgets_map[widget_hash]
             log.debug("All instances of {cls} with hash {widget_hash!r} have been deleted"
                 .format(cls=class_, widget_hash=widget_hash))
+            self.host.callListeners("widgetDeleted", widget_to_delete)
 
 
 class QuickWidget(object):