Fix issue with our tag property override and the 10.10 SDK

Until the 10.10 SDK we could use the compiler’s synthesized getter for our ‘tag’ property, but NSView.h in the 10.10 SDK declares ‘tag’ via the @property syntax and it is effectively unsupported to redeclare a readonly property as readwrite in a subclass (clang will not synthesize getter/setter methods in the subclass).