AK: Rename Vector::append(Vector) => Vector::extend(Vector)

Let's make it a bit more clear when we're appending the elements from
one vector to the end of another vector.

Userland/Libraries/LibCore/EventLoop.h

@@ -58,7 +58,7 @@ public:
 
     void take_pending_events_from(EventLoop& other)
     {
-        m_queued_events.append(move(other.m_queued_events));
+        m_queued_events.extend(move(other.m_queued_events));
     }
 
     static void wake();