Simplify love.audio's Pool class

Move most logic back to Source. Pool now simply contains a list of playing
sources and a mutex. Also gets rid of the pause/pauseAtomic duplication in
Source, since it can now request a lock from Pool itself.

--HG--
branch : minor

src/modules/thread/threads.h

@@ -57,6 +57,7 @@ class Lock
 public:
 	Lock(Mutex *m);
 	Lock(Mutex &m);
+	Lock(Lock &&other);
 	~Lock();
 
 private: