Inherit from QOpenGLWidget instead of QGLWidget

QGLViewer extends QOpenGLWidget instead of the deprecated QGLWidget (huge thanks to Gianluca).

• Although there are minor differences between the two classes, the switch should be mostly invisible.
• Using update() instead of updateGL() is one of them.
• The renderText methods were backported to maintain backward compatibility.
• Some overloaded constructors (using a QGLFormat or a sharedWidget) are deprecated since they have no equivalent.