Use power of LazyInitialise to replace ObserverMixin.__init__()

lib/tfw/mixins/observer_mixin.py

@@ -3,10 +3,13 @@
 
 from watchdog.observers import Observer
 
+from tfw.decorators import LazyInitialise
+
 
 class ObserverMixin:
-    def __init__(self):
-        self.observer = Observer()
+    @LazyInitialise
+    def observer(self):
+        return Observer()
 
     def watch(self):
         self.observer.start()