Qt widget: migrate QGLWidget object to QOpenGLWidget introduced in Qt 5.4

Qt 5.4 introduces a new object called QOpenGLWidget to replace QGLWidget used in Monteverdi.

The difference is described in qt documentation:

http://doc.qt.io/qt-5/qopenglwidget.html#differences-to-qglwidget

It brings new features, fix some issues with double screen (see https://stackoverflow.com/questions/33545006/qt5-attempt-to-set-a-screen-on-a-child-window-many-runtime-warning-messages#34066254)

I think that QGLWidget is now deprecated in Qt.