macosx: Fix issues with initial state of library window navigation stack

This PR tackles two bugs with the navigation stack:

1. Navigation buttons being incorrectly enabled on initial window load, when no navigation has taken place
2. The restoration of the library window state incorrectly setting the navigation state for the library window navigation stack (dual entries, some not being correctly entered)

Signed-off-by: Claudio Cambra claudio.cambra@gmail.com