title: Tkcalendar clear DateEntry after moving focus
body: <p>I have a DateEntry widget and a button to clear the DateEntry contents. I want the DateEntry to be blank unless the user selects a date. </p>
  
  <p>My problem is if I select the DateEntry widget and give it focus, then using the clear button doesn't work the first time because the DateEntry is auto-completing after losing focus. </p>
  
  <p>Example: </p>
  
  <pre><code>from tkinter import *
  from tkinter import ttk
  from tkcalendar import DateEntry
  
  def clearDateEntry():
      clearButton.focus_set()
      myDateEntry.delete(0,END)
  
  root=Tk()
  
  myDateEntry=DateEntry(root)
  myDateEntry.pack()
  myDateEntry.delete(0,END)
  
  clearButton=Button(root,text='clear',command=clearDateEntry)
  clearButton.pack()
  
  root.mainloop()
  </code></pre>
  
  <p>To replicate: give DateEntry focus, then click clear.</p>
  
  <p>In the <code>clearDateEntry()</code> function, I move the focus to the clear button first which I'd expect would trigger the auto-complete so that I can delete it straight after, but that doesn't work.</p>
  
  <p>How do I make <code>clearDateEntry()</code> actually clear the DateEntry on the first go? If there's a way to turn off DateEntry's auto-complete, that would also solve my problem.</p>