Remove assumption from tab widget that changes to children should reset selected index to 0.

It will be the user’s responsibility to set the selected index to something that makes sense when they change the children.

Author: jasongrout

ipywidgets/widgets/widget_selectioncontainer.py

@@ -90,11 +90,6 @@ def __init__(self, **kwargs):
             kwargs['selected_index'] = 0
         super(Tab, self).__init__(**kwargs)
 
-    @observe('children')
-    def _observe_children(self, change):
-        if len(change.new) > 0:
-            self.selected_index = 0
-
 @register
 class Stacked(_SelectionContainer):
     """Displays only the selected child."""