First Widget tutorial - use class variables

* Correct whitespace issues in 6_button.py
* Make "painter" a class variable (self.painter)
* Reference `self.clear_canvas` when binding the clear button callback
* Call `self.painter.canvas.clear()` in button callback
* Adjust the number of parameters for callback method

examples/guide/firstwidget/6_button.py

@@ -23,17 +23,16 @@ class MyPaintApp(App):
 
     def build(self):
         parent = Widget()
-        painter = MyPaintWidget()
+        self.painter = MyPaintWidget()
         clearbtn = Button(text='Clear')
-        parent.add_widget(painter)
+        clearbtn.bind(on_release=self.clear_canvas)
+        parent.add_widget(self.painter)
         parent.add_widget(clearbtn)
-
-        def clear_canvas(obj):
-            painter.canvas.clear()
-        clearbtn.bind(on_release=clear_canvas)
-
         return parent
 
+    def clear_canvas(self, obj):
+        self.painter.canvas.clear()
+
 
 if __name__ == '__main__':
     MyPaintApp().run()