Support for NSTabViewItems with view controllers under 10.10+.

[mgriebling]

It seems that the identifier field in the NSTabViewItem is used for storing the associated tab's view controller under current OSX releases 10.10+ which conflicts with storing the MMTabBarItem there. Does anyone have a work-around for this? In particular this happens when one assigns to the viewController property of NSTabViewItem. Alternatively, is there a way to have a view controller manage the tab's view without using the viewController property?