Add settings to control framerate when game is not in focus or minimised

Currently when Veloren is not focused or is minimised, it will continue to run at the maximum framerate set in the graphics settings. When you're not looking at the game (or can't even see it!), there's no real reason for it to run at that framerate. Other games I've played either come with a tickbox or a slider to control the maximum framerate in these cases.

When the game is minimised, it would be great if the framerate is limited further or rendering is disabled.

This would be great for portable devices like laptops and gaming handhelds which currently will kick up their fans when the game is running in the background and waste battery.