Can't be embedded in NSViewController.

[listeningwing]

Two serious issue made this excellent component unuseful.

1. It only support NSWindowController but can't be embedded in NSViewController that made it unavailable for most apps that requires customised NSView working with NSViewController.

2. NSTabViewItems must associated with a NSViewController in most situations, see Support for NSTabViewItems with view controllers under 10.10+. #54, (Support for NSTabViewItems with view controllers under 10.10+).

Test environment:
Xcode 10 on High Sierra (10.13).

Thanks for such an excellent component, but it's not perfect, still needs improvement.