LibGUI: Make GSocket connection asynchronous.

Now connect() will return immediately. Later on, when the socket is actually
connected, it will call GSocket::on_connected from the event loop. :^)

LibGUI/GNotifier.h

@@ -18,6 +18,7 @@ public:
 
     int fd() const { return m_fd; }
     unsigned event_mask() const { return m_event_mask; }
+    void set_event_mask(unsigned event_mask) { m_event_mask = event_mask; }
 
 private:
     int m_fd { -1 };