[Actor] Call stop() in Actor's destructor

While this does not fix anything, it appears sensible that we call
stop() prior destruction.

emper/Actor.hpp

@@ -57,7 +57,7 @@ class Actor {
 	void stop() { setState(Stopped); }
 
  public:
-	virtual ~Actor() = default;
+	virtual ~Actor() { stop(); }
 
 	template <CallerEnvironment callerEnvironment = CallerEnvironment::EMPER>
 	void start() {