[Development] Late API addition in QScreen for 5.0

[Samuel Rødal]

Hello,

to be able to achieve smooth animations in QML 2, the animations should
ideally use a fixed timestep, and not a timer which might have
inaccuracies depending on the platform and won't give fully smooth results.

In the context of this, and to avoid having values of 60 hard-coded (we
have a couple of those in qtdeclarative/src/quick already), we really
need to know the refresh rates of the screens we're rendering to. I've
therefore added QScreen::refreshRate(), which gives the screen's
vertical refresh rate, and thus can be used to run visual animations
with a fixed timestep without the animation running faster or slower on
different displays.

The change is up for review at
https://codereview.qt-project.org/#change,25317

--
Samuel