BLooper: Add DispatchExternalMessage()

* This is primarily a service method for ports of widget tool kits
  that require single-threaded GUI. DispatchExternalMessage() calls
  DispatchMessage(), but also sets fLastMessage, so that
  [Detach]CurrentMessage() work correctly. This allows to detach a
  message in DispatchMessage() when called from the window thread,
  add it to a global queue, and later process the queued messages in
  a different thread that calls DispatchExternalMessage().
* BLooper/BWindow: Make sure fLastMessage is accessed only when locked.

headers/os/app/Looper.h

@@ -52,6 +52,8 @@ public:
 	virtual	void			MessageReceived(BMessage* message);
 			BMessage*		CurrentMessage() const;
 			BMessage*		DetachCurrentMessage();
+			void			DispatchExternalMessage(BMessage* message,
+								BHandler* handler, bool& _detached);
 			BMessageQueue*	MessageQueue() const;
 			bool			IsMessageWaiting() const;