Deprecate clutter_threads_init()

GLib deprecated g_thread_init(), and threading support is initialized
by GObject, so Clutter already runs with threading support enabled. We
can drop the clutter_threads_init() call requirement, and initialize the
Big Clutter Lock™ on clutter_init(). This reduces the things that have
to be done when dealing with threads with Clutter, and the things that
can possibly go wrong.

tests/interactive/test-threads.c

@@ -188,8 +188,6 @@ test_threads_main (int argc, char *argv[])
     { 400, 150 }
   };
 
-  g_thread_init (NULL);
-  clutter_threads_init ();
   if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
     return 1;