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 <axeld@pinc-software.de>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/user/app/Looper.dox

@@ -1,14 +1,15 @@
 /*
- * Copyright 2008-2014 Haiku, Inc. All rights reserved.
+ * Copyright 2008-2018 Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  *
  * Authors:
  *		Niels Sascha Reedijk, niels.reedijk@gmail.com
  *		John Scipione, jscipione@gmail.com
+ *		Adrien Destugues, pulkomandy@pulkomandy.tk
  *
  * Corresponds to:
- *		headers/os/app/Looper.h	hrev47355
- *		src/kits/app/Looper.cpp	hrev47355
+ *		headers/os/app/Looper.h	hrev52501
+ *		src/kits/app/Looper.cpp	hrev52501
  */
 
 
@@ -603,6 +604,20 @@
 */
 
 
+/*!
+	\fn thread_id BLooper::Loop()
+	\brief Run the event loop in the current thread.
+
+	This method runs the event loop in an already existing thread. It blocks
+	until the looper stops looping. This can
+	be used to turn an existing thread into a BLooper.
+
+	Make sure the looper is not yet running before you call this method.
+
+	\since Haiku R1
+*/
+
+
 /*!
 	\fn void BLooper::Quit()
 	\brief Hook method that is called after a \c B_QUIT_REQUESTED message.

headers/os/app/Looper.h

@@ -69,6 +69,7 @@ public:
 
 	// Loop control
 	virtual	thread_id		Run();
+			void			Loop();
 	virtual	void			Quit();
 	virtual	bool			QuitRequested();
 			bool			Lock();

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;

src/kits/app/Looper.cpp

@@ -457,6 +457,24 @@ BLooper::Run()
 }
 
 
+void
+BLooper::Loop()
+{
+	AssertLocked();
+
+	if (fRunCalled) {
+		// Not allowed to call Loop() or Run() more than once
+		debugger("can't call BLooper::Loop twice!");
+		return;
+	}
+
+	fThread = find_thread(NULL);
+	fRunCalled = true;
+
+	task_looper();
+}
+
+
 void
 BLooper::Quit()
 {