Warn when subclassing NSCell without implementing Copy (NSZone) and an IntPtr constructor

NSCell is special (in a bad way): https://github.com/xamarin/maccore/commit/d762a2202ef87fa8407da5b92444dd4e144485ba

This means that any custom NSCell subclasses should provide an IntPtr constructor, and probably override Copy (NSZone) as well.

We should add a warning to mmp to tell users about this.

This is only an issue in Xamarin.Mac (Apple wisely never added the evil `NSCopyObject` method in iOS, so this problem will never show up there).

Ref 1: https://xamarinhq.slack.com/archives/C03CCJHCF/p1523012929000220
Ref 2: https://xamarinhq.slack.com/archives/C8RGY7DMX/p1523003662000529