Warn when factory tries to re-register an existing class with different bases

This fixes #1409

When running the following code a warning should now be issued:

    from kivy.app import runTouchApp
    from kivy.lang import Builder
    from kivy.uix.label import Label
    
    
    kv = '''
    MyClass
    
    <MyClass@TextInput+BoxLayout>
    '''
    
    
    class MyClass(Label):
        pass
    
    if __name__ == '__main__':
        runTouchApp(Builder.load_string(kv))

You'd also get a warning if you first loaded builder before the `MyClass` is decaled in py.