Cleaning up WarpPoint mess

 WarpPoint class has some issues with inheriting WorldPoint, because it introduces duplicate variables for position (Point.x and WarpPoint.xd, etc.).
Having duplicate variables will propably cause issues with data-backend too, because it will try to serialize both the `int x` as well as `double xd` variables.
This patch makes this class an independent from WorldPoint while maintaining all compability.
It also removes the public variables and uses getters and setters instead.

@luacs1998 What do you think? Another solution could be only making the fields private and patching the data-backend to customize the persisting of WarpPoint to exclude the int-values. Also the setters would then explicitly set the int-values when changing the double-ones and the same for the other way round.