title: Traits List handler does not react to += list extension
body: <pre><code>from traits.api import List, HasTraits
  
  class Foo(HasTraits):
     x = List
     def _x_items_changed(self,new):
        print new.added
  
  f = Foo()
  f.x = [1,3]
  f.x.append(9) #handler reacts!
  f.x += [9,10] # handler does not react! Why?
  </code></pre>
  
  <p>What am I missing here?</p>
  
  <p>Thanks!</p>