(PhVideoEngine) Fix framerate computation

There was a couple of issues related to integer division. A 23.97 fps
video was handled as a 23 fps video !
(PhClock) Fix ticking with 23.97 fps videos
Switch frequency to a real instead of integer

Screen refresh rate can be a float (59.something Hz), that is why Qt
returns it as a qreal. Use that in Joker too.
Merge pull request #273 from tlecomte/tlNonintegerFps

Tl noninteger fps