title: Kivy class factory error
body: <p>I tried to make a program with a screen manager an images you can click on.
  I first I tried to store the kivy file within a string variable and return the string variable, but I got this error message:</p>
  
  <pre><code> kivy.factory.FactoryException: Unknown class &lt;BILD1&gt;
  </code></pre>
  
  <p>So I tried to return the Screenmanager, but it did not seem to work.
  I still got the same error message, could you please help me.
  Here is the rest of my source code:</p>
  
  <pre><code>class Auswahl(Screen):
      pass
  class Frage(Screen):
      farbe = ListProperty([1, 1, 1, 1])
      def druck(self):
          self.farbe = ([1, 0, 0, 1])
          self.ids.box1.clear_widgets()
          wimg = Image(source='Bild1.png')
          self.ids.box1.add_widget(wimg)
  
  class Troll(Screen):
      pass
  
  class Manager(ScreenManager):
      pass
  
  Builder.load_file('turf.kv')
  
  class BILD1(Widget):
      velocity = ListProperty([1, 0])
      def __init__(self, **kwargs):
          super(Knopf, self).__init__(**kwargs)
          Clock.schedule_interval(self.Update, 1/60.)
      def Update(self, *args):
          pass
      def on_touch_down(self, touch):
          if self.collide_point(*touch.pos):
              print 'es geht'
  class BILD2(Knopf):
      def on_touch_down(self, touch):
          if self.collide_point(*touch.pos):
              print 'es geht'
  class BILD3(Knopf):
      def on_touch_down(self, touch):
          if self.collide_point(*touch.pos):
              print 'es geht'
  class BILD4(Knopf):
      def on_touch_down(self, touch):
          if self.collide_point(*touch.pos):
              print 'es geht'
  
  
  
  class TurF(App):
      def build(self):
          return Manager()
  
  TurF().run()
  </code></pre>