LibCore: Make Core::Timer::create_single_shot() create a stopped timer

None of the code using this actually expected the timer to fire right
away, but they would instead call start() on it once they were ready to
accept a timer fire.

Let's make the API behave the way its clients believed it did. :^)

Libraries/LibCore/Timer.h

@@ -39,6 +39,7 @@ public:
     {
         auto timer = adopt(*new Timer(interval, move(timeout_handler), parent));
         timer->set_single_shot(true);
+        timer->stop();
         return timer;
     }