title: How i call another "window" in C with GTK3
body: <p>i want to call another window when i press the button 
  "on_btn_janelaCriarSessao_clicked", my code of the window is this:</p>
  
  <pre><code>JanelaCriarSessao::JanelaCriarSessao(){
  /* JANELA CRIAR SESSAO */
  
  windowCriarSessao = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  //
  gtk_window_set_default_size(GTK_WINDOW(windowCriarSessao), 400, 250);
  gtk_window_set_title(GTK_WINDOW(windowCriarSessao), "Criar Sessao");
  gtk_window_set_position(GTK_WINDOW(windowCriarSessao), GTK_WIN_POS_CENTER_ALWAYS);
  gtk_window_set_resizable(GTK_WINDOW(windowCriarSessao), FALSE);
  
  textoInformativo = gtk_label_new("Digite a chave(ID) da sessao abaixo: ");
  entry_IDSESSAO = gtk_entry_new ();
  btn_cadastrarSessao = gtk_button_new_with_label("Cadastrar");
  
  caixaWidgets = gtk_box_new(GTK_ORIENTATION_VERTICAL, 5);
  
  //empacotamento dos widgets na janela
  gtk_box_pack_start(GTK_BOX(caixaWidgets), textoInformativo, TRUE, TRUE, 3);
  gtk_box_pack_start(GTK_BOX(caixaWidgets), entry_IDSESSAO, TRUE, TRUE, 3);
  gtk_box_pack_end(GTK_BOX(caixaWidgets), btn_cadastrarSessao, TRUE, TRUE, 3);
  
  g_signal_connect (GTK_ENTRY(entry_IDSESSAO), "activate", 
      G_CALLBACK(entry_activate), textoInformativo);
  
  //Connects GCallback function gtk_main_quit to "destroy" signal for "window"
  g_signal_connect(G_OBJECT(windowCriarSessao), "destroy", 
      G_CALLBACK(gtk_main_quit), NULL);
  
  gtk_container_add(GTK_CONTAINER(windowCriarSessao), caixaWidgets);}
  </code></pre>
  
  <p>and the code of button is this:</p>
  
  <pre><code>void JanelaPrincipal::mostrar(){
  //mostra a janela principal
  gtk_widget_show_all(window);} 
  void on_btn_janelaCriarSessao_clicked(GtkWidget *widget, JanelaPrincipal *data){ JanelaCriarSessao obj;
  obj.mostrar();}
  </code></pre>
  
  <p>But, when i press the button, he open the window but without any widget, a blank Window just with the title i put in.
  The code of my main.cpp:</p>
  
  <pre><code>gtk_init(&amp;argc, &amp;argv);
  
  JanelaPrincipal obj;
  obj.mostrar();
  
  gtk_main();
  
  return 0;}
  </code></pre>