title: Two kivy Images in an BoxLayout
body: <p>I tried to make an kivy program wit two images , which are supposed to change when I click on them.
  But when I tred to add two Widgets with the containing Images to an BoxLayout, I got one image at the position 0, 0
  Why dosen't the BoxLayout work with my images?
  Here is my code:</p>
  
  <pre><code>from kivy.app import App
  from kivy.uix.widget import Widget
  from kivy.lang import Builder
  from kivy.core.window import Window
  from kivy.properties import NumericProperty
  from kivy.properties import StringProperty
  from kivy.uix.button import Button
  from kivy.uix.boxlayout import BoxLayout
  
  class Feld(Widget):
      droga = StringProperty('hinterg.png')
      kr = StringProperty('kreuz.png')
      ks = StringProperty('kreis.png')
      def on_touch_down(self, touch):
          if self.collide_point(*touch.pos):
              self.droga = self.kr
  
  root = Builder.load_string('''
  BoxLayout:
      orientation: 'horizontal'
      Feld:
          id: a1
      Feld:
          id: a2
  
  &lt;Feld&gt;:
      Image:
          source: root.droga
  
  
  
  ''')
  
  class app(App):
      def build(self):
          Window.clearcolor = (0, 0.54, 1, 1)
          return root
  
  if __name__ == "__main__":
      app().run()
  </code></pre>