VideoReferenceClock: make refresh rate floating point + cleanup

I am tired of explaining "why does XBMC speed up my video". Users are confused by the rounded value of refresh rate. On Linux refresh rate is detected by 

```
m_RefreshRate = g_graphicsContext.GetFPS()
```

means that GraphicsContext always knows the current refresh rate. Any further detection of refresh rate  in videorefclock is unneeded and messy. 

I encourage other platform devs to do some clean-up too. According to the forums measurement of refresh rate on Windows is utterly broken (g_advancedSettings.m_measureRefreshrate)