Refactor callback subscription stuff to a mixin

lib/tfw/components/mixins/callback_mixin.py

@@ -0,0 +1,17 @@
+from functools import partial
+
+
+class CallbackMixin:
+    def __init__(self):
+        self._callbacks = []
+
+    def subscribe_callback(self, callback, *args, **kwargs):
+        fun = partial(callback, *args, **kwargs)
+        self._callbacks.append(fun)
+
+    def unsubscribe_callback(self, callback):
+        self._callbacks.remove(callback)
+
+    def _execute_callbacks(self, *args, **kwargs):
+        for callback in self._callbacks:
+            callback(*args, **kwargs)