iOS: Add standard paths implementation for iOS

A standard paths implementation for iOS. The APIs used on iOS require
that the source file be "Objective-C++" (with a .MM extension), as
there are no APIs available in C++ for this. The implementation
complies with the latest documentation on standard paths.

Change-Id: I349d3c5d4ddb6fb1297a45dc9ae26b56ac528abb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: David Faure <david.faure@kdab.com>