Qt5.4 QApplication no such file or directory

Qt5.4 for Windows 

Error c1083: Cannot open include file: QApplication: no such file or directory

範例為:  MDI Example

 http://doc.qt.io/qt-5/qtwidgets-mainwindows-mdi-example.html

原文在 http://qt-project.org/forums/viewthread/23804

修改 *.pro檔案

  QT += qml quick widgets

 或者是 

  QT += core gui widgets