BLooper: API to hijack existing thread.

I need this to use loopers in WebKit, which spawns threads and expects
to be able to turn them into event loops later on.

This is the pattern already used in BApplication, we may as well make it
available elsewhere.

Change-Id: I5939ca89d33cb3bcc92567b302c2038d976af598
Reviewed-on: https://review.haiku-os.org/735
Reviewed-by: Axel Dörfler <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

src/kits/app/Application.cpp

@@ -590,15 +590,7 @@ BApplication::Run()
 	if (fInitError != B_OK)
 		return fInitError;
 
-	AssertLocked();
-
-	if (fRunCalled)
-		debugger("BApplication::Run was already called. Can only be called once.");
-
-	fThread = find_thread(NULL);
-	fRunCalled = true;
-
-	task_looper();
+	Loop();
 
 	delete fPulseRunner;
 	return fThread;