title: BoxLayout draw all widget in the corner
body: <p>I'm trying to draw different widget in a box layout.
  But if I try to draw some class, which extend widget, implemented by myself, the BoxLayout draw everything in the low left corner.
  Trying to change size_hint and the size of the root didn't give any result.</p>
  
  <pre><code>BoxLayout:  
      orientation: 'horizontal'
      Button:
          text: 'a'
      Label:
          text: 'b'
      Button:         
          text: 'c'
      TextInput:
          text: 'd'
  </code></pre>
  
  <p>Works</p>
  
  <pre><code>BoxLayout:  
      orientation: 'horizontal'
      Widget:
          Button:
              text: 'a'
      Widget:
          Label:
              text: 'b'
  </code></pre>
  
  <p>gives problem.</p>
  
  <p>Any ideas?</p>