Implemented direct message passing for local targets; this fixes a deadlock

with PostMessage() in case the message queue is full.
Some notes:
* for synchronous replies, we don't use this mechanism yet, but it could be
  extended to do that as well.
* the code looks so complicated because we need a way to access the looper's
  queue without locking it (to prevent deadlocks); like Dano's solution, I've
  abused BTokenSpace to store a BDirectMessageTarget with a BHandler.
* we also need to decouple the lifetime of a looper's queue from its target,
  as we cannot lock the looper, and therefore, can't guarantee it stays valid
  as long as we're accessing it outside of BLooper.
* init_clipboard() now needs to be done after the global constructors have
  been called - since sending messages now needs gDefaultTokens to be initialized.
  Since this is done per image, it shouldn't cause any troubles, though.
* some minor cleanup, removed unused _msg_cache_cleanup_() and friends.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19968 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/os/app/Looper.h

@@ -18,6 +18,7 @@
 class BMessage;
 class BMessageQueue;
 namespace BPrivate {
+	class BDirectMessageTarget;
 	class BLooperList;
 }
 
@@ -150,7 +151,7 @@ private:
 			BHandler*		resolve_specifier(BHandler* target, BMessage* msg);
 			void			UnlockFully();
 
-			BMessageQueue*	fQueue;
+			BPrivate::BDirectMessageTarget* fDirectTarget;
 			BMessage*		fLastMessage;
 			port_id			fMsgPort;
 			int32			fAtomicCount;