Added QScreen::refreshRate() to get the vertical refresh rate.

Added QScreen::refreshRate() to get the vertical refresh rate.

To give applications that want it the option to use a fixed timestep for
animations, and to avoid having values of 60 hard-coded (we have a
couple of those in qtdeclarative/src/quick already), we need to know the
refresh rates of the screens we are rendering to.

Change-Id: Ife49162e830440ad7eab563a27e8aebbbafc5fc5
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>