GTask: Avoid resorting

When a task is cancelled, we want to move it to the front
of the queue - our sort function does that for us, but there
is no need to resort the entire queue here, we can just
move the one item and be done with it. This uses just-introduced
threadpool api for this purpose.

https://bugzilla.gnome.org/show_bug.cgi?id=751160

gio/gtask.c

@@ -1296,7 +1296,10 @@ task_thread_cancelled (GCancellable *cancellable,
 {
   GTask *task = user_data;
 
-  g_task_thread_pool_resort ();
+  /* Move this task to the front of the queue - no need for
+   * a complete resorting of the queue.
+   */
+  g_thread_pool_move_to_front (task_pool, task);
 
   g_mutex_lock (&task->lock);
   task->thread_cancelled = TRUE;