Children fire event

Now when a container widget is displayed, the _handle_displayed method
of each of its children is fired.

IPython/html/widgets/widget_container.py

@@ -27,6 +27,15 b' class ContainerWidget(DOMWidget):'
     children = List(Instance(DOMWidget))
     _children = List(Instance(DOMWidget), sync=True)
 
+
+    def __init__(self, **kwargs):
+        super(ContainerWidget, self).__init__(**kwargs)
+        self.on_displayed(ContainerWidget._fire_children_displayed)
+
+    def _fire_children_displayed(self):
+        for child in self._children:
+            child._handle_displayed()
+
     def _children_changed(self, name, old, new):
         """Validate children list.