Provide a way to run animations at half a speed

On low-end embedded ARM hardware drawing and running any animations at 60fps on Qt Quick window can hike up the CPU usage to 40%, which is too much in our use case. Also it is often better to run smooth 30fps than attempt to support a higher frame rate that ends up oscillating between 40-60fps on the low-end hardware, causing unwanted flicker on the screen.

AFAICS currently Qt Quick refresh rate is hardcoded to the screen refresh rate provided by the platform plugin. While there are no use cases to run the screen refresh rate higher than the GUI, limiting the physical refresh rate can be problematic, cause bizarre timing issues. Also in our case the system needs to different display configurations.