Fix is_extended method for UI classes (menu, panel, headers... etc)

Appending then removing a draw function caused the is_extended()
method to return true afterwards, resolve by checking if there are
multiple draw functions.

scripts/modules/bpy_types.py

@@ -1044,7 +1044,12 @@ class _GenericUI:
 
     @classmethod
     def is_extended(cls):
-        return bool(getattr(cls.draw, "_draw_funcs", None))
+        draw_funcs = getattr(cls.draw, "_draw_funcs", None)
+        if draw_funcs is None:
+            return False
+        # Ignore the first item (the original draw function).
+        # This can happen when enabling then disabling add-ons.
+        return len(draw_funcs) > 1
 
     @classmethod
     def append(cls, draw_func):