Move QString <-> NSString conversion to QtCore

Move QString <-> NSString conversion to QtCore

Add (private) API to QCFString:
	static QString toQString(NSString *)
	static NSString *toNSString(const QString &)

Add implementation to qcore_mac_objc.mm. Keep the
mac_cpp since it's used for building qmake as well
as bootstrapping.

Replace usage of NSString conversion functions
in the cocoa and corewlan plugin with QCFString.

Change-Id: I9f34edd5231255aef9d8d6e9a60306174bb279b3
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>