Implement datarefs to report map coordinates, zoom and range

[mjh65]

This PR implements the enhancement requested in https://github.com/fpw/avitab/issues/198.

The first commit refactors some existing code to put a small wrapper class around the X-Plane API calls. Despite this, the callbacks are still a little more 'C' than 'C++' (reinterpret casts are used), and perhaps could be improved by some more complex (and less readable?) template code.

The second commit adds 4 new datarefs to allow other X-Plane components to obtain the Avitab map coordinates (map centre), the zoom level, and an approximation of the distance from top to bottom of the visible map.