Fixed: another tab view decoding error.

Without this fix, CPTabView would still make view hierarchy changes in `initWithCoder:` despite the fix in f805b6b. Making view hierarchy changes before a cib is fully decoded can easily result in a corrupted hierarchy.

In this case, the decoding of a tab view item view, A, lead to the decoder to try to decode A's next responder, which was the tab view, B. B then tried to set the selected tab, which involved adding A as a subview. But since A wasn't done decoding, A._superview would change later in the process, corrupting this change.

This fix delays `_updateItems` to `awakeFromCib` at which time changing the view hierarchy is safe.

Refs #1409.