Update of DemoWindow.xib

Set DemoWindowController as delegate of Demo Window
Update of MMTabDragAssistant

-dealloc now releases _pasteboard item if still around.
-_dragAttachedTabBarButton: ofTabBarView: at: event: source: now
invalidates stored _pasteboard item if not continuing with detached
drag. This method now creates the pasteboard item (had been created by
calling method)