PhotoManager: Check for IOErrors during opens.

When readingg a file, check for IOErrors. The ImageCache will use this
to signal a bad cache attempt.
Pages: Add Photo Settings, and remove last fake.

Add an area where Photo Settings will be located. The last fake part was
changed to the PhotoNext part.
Models: Remove fake model factory registrations.

Remove the fake model registrations to the factory - the same is now
achieved by appending "_empty" to the part name in the *.edc file.
PhotoManager: Don't allow duplicates in the cache.