CEventLoop: Don't re-process already processed events when un-nesting.

If we had already processed a couple of queued events by the time we were
told to un-nest the event loop, we'd put the entire current batch at the
head of the outer queue. This meant that we might end up trying to process
the same events multiple times.

Let's not do that. :^)

Libraries/LibCore/CEventLoop.cpp

@@ -141,8 +141,12 @@ void CEventLoop::pump(WaitMode mode)
 
         if (m_exit_requested) {
             LOCKER(m_lock);
-            // FIXME: Shouldn't we only prepend the events that haven't been processed yet?
-            m_queued_events.prepend(move(events));
+            decltype(m_queued_events) new_event_queue;
+            new_event_queue.ensure_capacity(m_queued_events.size() + events.size());
+            for (; i < events.size(); ++i)
+                new_event_queue.unchecked_append(move(events[i]));
+            new_event_queue.append(move(m_queued_events));
+            m_queued_events = move(new_event_queue);
             return;
         }
     }