Add center() function to QGeoShape.

It is frequently useful to calculate the geometric center of a shape.
Both QGeoCircle and QGeoRectangle already define center() functions,
however, the application developer must cast each QGeoShape into either
a QGeoCirlce or QGeoRectangle before calling. Providing
QGeoShape::center() allows application code to be simplified.

Existing QGeoCircle::center() and QGeoRectangle::center() functions are
kept for compatibility.

Change-Id: I92b727ab5e713f70174588a27040446c992ae14e
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>