[patch] Using NSMenuItem class instead of NSMenuItem protocol

Howdy!

There's a lot of Cocoa's deprecated methods being used in current
version of AbiWord.
So, I'm starting a big clean to fix possible problems and segmentation faults.

I'm trying to organize my self, so, some patches will be send (I hope)
each one fixing a specific problem.

First of this is to use NSMenuItem class instead of NSMenuItem
protocol, according with developer.apple.com:

"The NSMenuItem protocol is being removed from the Application Kit;
you must use the
NSMenuItem class instead."

So, could someone review and apply?

Best Regards,

-- 
Fabiano Fidêncio