Use power of LazyInitialise to replace CallbackMixin.__init__()

lib/tfw/components/history_monitor.py

@@ -36,7 +36,6 @@ class HistoryMonitor(CallbackMixin, ObserverMixin, ABC):
     See examples below.
     """
     def __init__(self, histfile):
-        CallbackMixin.__init__(self)
         self.histfile = histfile
         self._history = []
         self._last_length = len(self._history)

lib/tfw/fsm_base.py

@@ -19,7 +19,6 @@ class FSMBase(CallbackMixin):
     states, transitions = [], []
 
     def __init__(self, initial: str = None, accepted_states: List[str] = None):
-        CallbackMixin.__init__(self)
         self.accepted_states = accepted_states or [self.states[-1]]
         self.machine = Machine(model=self,
                                states=self.states,

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):
         """