Thread system and blockingActions

* BlockingAction is a subclass from Action, whose exec method stops the thread until a signal is received (so a subclass from BlockingAction could stop it by calling its parent's exec method). This signal comes from the method manager.notifystart(Node), which can be called from LUA.

* A thread is created for each manager.notify call, so that each execution chain gets executed in a separate thread, and stopping it doesn't stop the whole execution.