title: How can I set the position (x,y coordinates) of a grid layout in kivy?
body: <p>I've noticed that when you make buttons using the grid layout in Kivy they are created at (0,0) and move over a number of spaces depending on the length and width of previous buttons. 
  However, I'd like to have a 3x4 grid in the middle of the bottom of the screen.</p>
  
  <p>I have this so far:</p>
  
  <pre><code>import kivy
  from kivy.uix.gridlayout import GridLayout
  from kivy.app import App
  from kivy.uix.button import Button
  
  class CalcApp(App):
      def build(self):
          layout = GridLayout(cols=3, row_force_default=True, row_default_height=50)
          layout.add_widget(Button(text='1', size_hint_x=None, width=100))
          layout.add_widget(Button(text='2', size_hint_x=None, width=100))
          layout.add_widget(Button(text='3', size_hint_x=None, width=100))
          layout.add_widget(Button(text='4', size_hint_x=None, width=100))
          layout.add_widget(Button(text='5', size_hint_x=None, width=100))
          layout.add_widget(Button(text='2', size_hint_x=None, width=100))
          layout.add_widget(Button(text='6', size_hint_x=None, width=100))
          layout.add_widget(Button(text='7', size_hint_x=None, width=100))
          layout.add_widget(Button(text='8', size_hint_x=None, width=100))
          layout.add_widget(Button(text='9', size_hint_x=None, width=100))
          layout.add_widget(Button(text='0', size_hint_x=None, width=100))
          layout.add_widget(Button(text='Enter', size_hint_x=None, width=100))
          return layout
  
  CalcApp().run()
  </code></pre>
  
  <p>So, how do I shift the position?</p>