title: Get return value in QML from Python method
body: <p>I'm trying to call a Python method from QML and use the return value.</p>
  
  <p>QML receives undefined, from the Python method.</p>
  
  <p>When passed back to Python it is just an empty string.</p>
  
  <pre><code>import sys
  from PyQt5.QtCore import QObject, pyqtSlot
  from PyQt5.QtWidgets import QApplication
  from PyQt5.QtQml import QQmlApplicationEngine
  
  
  class InPython(QObject):
      @pyqtSlot(str, )
      def login(self, Login):
          print(Login)
          return "a"
  
  
  if __name__ == "__main__":
      app = QApplication(sys.argv)
      engine = QQmlApplicationEngine()
      context = engine.rootContext()
      context.setContextProperty("main", engine)
      engine.load('Main.qml')
      win = engine.rootObjects()[0]
  
      inPython = InPython()
      context.setContextProperty("loginManger", inPython)
  
      win.show()
      sys.exit(app.exec_())
  </code></pre>
  
  <p>Main.qml</p>
  
  <pre><code>import QtQuick 2.3
  import QtQuick.Controls 1.2
  import QtQuick.Layouts 1.0
  
  ApplicationWindow {
      width: 800;
      height: 600;
  
      ColumnLayout {
          anchors.horizontalCenter: parent.horizontalCenter
          anchors.verticalCenter: parent.verticalCenter
          anchors.margins: 3
          spacing: 3
          Column {
              spacing: 20
              anchors.horizontalCenter: parent.horizontalCenter
  
              TextField {
                  id: login
                  objectName: "login"
                  placeholderText: qsTr("Login")
                  focus: true
                  Layout.fillWidth: true
                  onAccepted: {
                      btnSubmit.clicked()
                  }
              }
  
              Button {
                  id: btnSubmit
                  objectName: "btnSubmit"
                  text: qsTr("Login")
                  Layout.fillWidth: true
                  onClicked: {
                      var a = loginManger.login(login.text);
                      console.log(a);
                      loginManger.login(a); // Python recieves ''
                      if (a === "a"){
                          login.text = "SUCCESS"
                      }
                  }
              }
          }
      }
  }
  </code></pre>