QGeoAreaMonitor should be able to report the minimum radius which it can work with

With the present API, if the user sets the radius of an area monitor to be less than the implementation-defined minimum radius the minimum radius will be used in place of the user defined radius.
The API should have a read only property so that client code can discover what the implementation-defined minimum radius is.

At present this may be possible (indirectly) by reading the radius after setting it and comparing the values, but we in the cases where the implementation-defined minimum radius is used instead of the users value we currently don't guarantee that this change will be propogated back to the radius() method.