LibCore: Convert CTCPSocket to ObjectPtr, add construct() helper

The C_OBJECT macro now also inserts a static construct(...) helper into
the class. Now we can make the constructor(s) private and instead call:

    auto socket = CTCPSocket::construct(arguments);

construct() returns an ObjectPtr<T>, which we'll later switch to being
a NonnullRefPtr<T>, once everything else in in place for ref-counting.

Libraries/LibGUI/GEventLoop.h

@@ -11,6 +11,7 @@ class CNotifier;
 class GWindow;
 
 class GWindowServerConnection : public IPC::Client::Connection<WSAPI_ServerMessage, WSAPI_ClientMessage> {
+    C_OBJECT(GWindowServerConnection)
 public:
     GWindowServerConnection()
         : Connection("/tmp/wsportal")