ListFragment, ListCursorAdapter: Prevent navigation to not-done items.

Because it is too tempting to skip ahead to only the interesting images.
As suggested here:
https://plus.google.com/100123542880051296706/posts/WcjVd8zKT7e
About: Add SDSS and Hubble image attribution.

As suggested here:
https://plus.google.com/100123542880051296706/posts/WcjVd8zKT7e