title: Finding the currently selected tab of Ttk Notebook
body: <p>I have a Ttk Notebook widget containing 8 Frames - so, 8 tabs. Each frame contains a Text widget. I have a button outside the Notebook widget, and I want to insert text into the current tabs Text widget when this button is pressed.</p>
  
  <p>This would seem to require working out which widget in the Notebook is currently selected, but I can't seem to find how to do this. How would I find the currently selected tab?</p>
  
  <p>Alternatively, how can I implement what I want to?</p>
  
  <p>If it helps, here's the code for my notebook:</p>
  
  <pre><code>self.nb = Notebook(master)
  self.nb.pack(fill='both', expand='yes', padx=10, pady=10)
  self.frames = []
  self.texts = []
  for i in xrange(8):
    self.frames.append(Frame())
    self.nb.add(self.frames[i])
    self.texts.append(Text(self.frames[i]))
    self.texts[i].pack(fill='both')
  </code></pre>