title: Select all in a tkinter Listbox
body: <p>I am creating a <code>Listbox</code> using Tkinter and Python. I want to make a Button for <code>select all</code>, but I can't find any info regarding selecting elements using code.</p>
  
  <pre><code> self.l = Listbox(self, height=12, selectmode=MULTIPLE)
   self.selectAll=Button(self, text="select all",
                        command=self.selectAllCallback())
   def selectAllCallback(self)
   # What to do here
  </code></pre>