Fix #2852: add optional 'property' parameter to getVehicleHandling function

[XJMLN]

Fixes #2852

Added optional 'property' argument to get particular vehicle handling value.

Usage:
getVehicleHandling(vehicle, "mass")

[botder]

I don't understand why the code (handling in general) has to "guess" what type each property is. All we need is an array for each entry in enum eHandlingProperty to tell you what the underlying type for the property is, because the type never changes.