Offer user quick restart if settings change

Ever been annoyed by those settings in the menu that require a restart? Well I haven't fixed those. But I have added a little dialog to save you some time! That's almost as good right?

![Restart Now?](http://imgur.com/HQJr5fT.png)

Hit the button and the game will be loading again within a second or two.

Doesn't check for mouse-lock in anticipation of #5597 landing.


These two flags aren't independent (`restart == true && quit == false` doesn't make sense), so it would make sense to replace these with an enum value.  You could perhaps rename `ReturnCode` -> `RunStatus` and add a `Running` state.