make control a cooperative object (#490)

I used the observer pattern to propagate changes in a usercontrol object. For that purpose my widget derives from UserControl and another class. Calling super().__init__() in my class did not work because I had the wrong order in the class calls, i.e., class MyWidget(UserControl, Subject), rather than class MyWidget(Subject, UserControl). By including super().__init__() in the Control class such points of confusion are easy to prevent. Here is a further discussion:

https://rhettinger.wordpress.com/2011/05/26/super-considered-super/

Author: ndvanforeest

sdk/python/flet/control.py

@@ -100,6 +100,7 @@ def __init__(
         disabled: Optional[bool] = None,
         data: Any = None,
     ):
+        super().__init__()
         self.__page: Optional[Page] = None
         self.__attrs = {}
         self.__previous_children = []