Refactor watchdog Observer handling to a mixin

lib/tfw/components/mixins/__init__.py

@@ -3,3 +3,4 @@
 
 from .supervisor_mixin import SupervisorMixin
 from .callback_mixin import CallbackMixin
+from .observer_mixin import ObserverMixin

lib/tfw/components/mixins/observer_mixin.py

@@ -0,0 +1,16 @@
+# Copyright (C) 2018 Avatao.com Innovative Learning Kft.
+# All Rights Reserved. See LICENSE file for details.
+
+from watchdog.observers import Observer
+
+
+class ObserverMixin:
+    def __init__(self):
+        self.observer = Observer()
+
+    def watch(self):
+        self.observer.start()
+
+    def stop(self):
+        self.observer.stop()
+        self.observer.join()