Simplified raycast API

* Added `World.rayCast` function, taking a `RayCast` object and the start and end points for the ray.
* Removed `RayCast.point1`, `RayCast.point2`, `RayCast.world`, `RayCast.repeat` and `RayCast.running`. All these properties can be trivially implemented on the QML side.
* Renamed `RayCast.result` to `RayCast.maxFraction`
* Renamed `RayCast.onCast` to `RayCast.onFixtureReported`
* Adjusted raycast example accordingly