Add a way to register loopers for quit

* BApplication can now take the job to quit a BLooper at
the application quit. It's rejecting requests from windows too.
* BMediaRoster is using now this service in conjunction with the
MediaRosterUndertaker.
* The BeBook specify that we should have a valid BApplication
before to instantiate the BMediaRoster. While in theory we should
add a debugger call when this situation happens, in pratice this
might lead to more problems. For example libraries might use the
media_kit and create a BApplication object, but they aren't
applications, this is a design problem. So I decided to replace it
with a TRACE call for the moment.

headers/os/app/Application.h

@@ -85,6 +85,11 @@ public:
 									BHandler* handler);
 			void				SetPulseRate(bigtime_t rate);
 
+	// Register a BLooper to be quit before the BApplication
+	// object is destroyed.
+			status_t			RegisterLooper(BLooper* looper);
+			status_t			UnregisterLooper(BLooper* looper);
+
 	// More scripting
 	virtual status_t			GetSupportedSuites(BMessage* data);