title: Position of widgets in GridLayout
body: <p>I'm trying to create a grid of widgets with each "cell" widget having a rectangle I can change the color of later.  When I run the code below with the line <code>grid.add_widget(Button(text=str(i)))</code> the buttons fill the window (as in the docs).  However, when I use my <code>Cell</code> widget as in <code>grid.add_widget(Cell())</code> all the widgets are bunched in the lower left corner (<code>position=[0,0]</code>, <code>size=[100,100]</code>).  I found this question <a href="https://stackoverflow.com/questions/30679110/kivy-add-raw-widget-to-gridlayout-as-opposed-to-image-button-etc">Kivy: Add raw Widget to GridLayout (as opposed to Image, Button, etc)</a> and changed my <code>Cell</code> class to inherit from <code>Layout</code> instead of <code>Widget</code> but that didn't help things.  Any ideas on what I'm doing wrong?  What I'd like is a grid of colored rectangles. </p>
  
  <p>Versions </p>
  
  <pre><code>[INFO   ] [Kivy        ] v1.9.1
  [INFO   ] [Python      ] v2.7.6 (default, Jun 22 2015, 17:58:13) 
  </code></pre>
  
  <p>Code</p>
  
  <pre><code>from kivy.app import App
  from kivy.core.window import Window
  from kivy.uix.widget import Widget
  from kivy.uix.button import Button
  from kivy.uix.gridlayout import GridLayout
  from kivy.uix.layout import Layout
  from kivy.graphics import Rectangle, Color
  
  XSIZE=2
  YSIZE=2
  
  class Cell(Layout):
      def __init__(self, *args, **kwargs):
          super(Cell, self).__init__(*args, **kwargs)
          with self.canvas:
              Rectangle(size=self.size, pos=self.pos)
              Color((0.2, 0.2, 0.2, 1.0))
  
  class GameApp(App):
      def build(self):
          grid = GridLayout(rows=YSIZE, cols=XSIZE, size=Window.size)
          for i in xrange(4):
              print 'i={}'.format(i)
  #            grid.add_widget(Cell())
              grid.add_widget(Button(text=str(i)))
          return grid
  
  if __name__ == '__main__':
      GameApp().run()
  </code></pre>