Use power of LazyInitialise to replace CallbackMixin.__init__()

lib/tfw/mixins/callback_mixin.py

@@ -3,10 +3,13 @@
 
 from functools import partial
 
+from tfw.decorators import LazyInitialise
+
 
 class CallbackMixin:
-    def __init__(self):
-        self._callbacks = []
+    @LazyInitialise
+    def _callbacks(self):
+        return []
 
     def subscribe_callback(self, callback, *args, **kwargs):
         """