unloadedWindowTitle is now defaultWindowTitle.

This makes it a bit more useful. It can be set via the property of the
same name, and will be used when:
- windowUsesViewControllerTitle is NO.
- A toolbar item doesn't have an identifier for some reason.
- A view controller has changed the title, and is removed. (That is, the
  title of the window will be reset when switching tabs, even if
  windowUsesViewControllerTitle is NO.)

After further investigation, I don't think the check for a changed view
controller inside of the block is actually wrong. Sorry for the false
accusations. :)