title: Centering BoxLayout with Buttons inside a GridLayout
body: <p>I'm working with kivy for a new project. While designing my GUI, I encountered the following problem.</p>
  
  <p>I have a GridView dividing my window in three section. The top sections contains a headline, and the middle section should contain centered Buttons and Labels.</p>
  
  <p>This is my current .kv file:</p>
  
  <pre><code>&lt;Main&gt;:    
  rows: 3
  
  Label:
      font_size: 25  
      text: "Some Headline"
  
  GridLayout:
      rows: 2
      row_force_default: True
      row_default_height: 40
      col_force_default: True
      col_default_width: 200
  
      BoxLayout:
          orientation: 'horizontal'
          Button:
              text: "Button 1"
          Label:
              text: "Label 1"
  
      BoxLayout:
          orientation: 'horizontal'
          Button:
              text: "Button 2"
          Label:
              text: "Label 2"
  </code></pre>
  
  <p>The resulting window looks like this</p>
  
  <p><img src="https://i.stack.imgur.com/pridm.png" alt=""></p>
  
  <p>But what I want instead is that these BoxLayouts with the Bottons/Labels are centered in the window.</p>
  
  <p>How can I accomplish this or what do I have to change?</p>