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Added the ability to load multiple camera backs.

This is a feature that required quite a bit of re-thinking of the structure of the app, but it is a vital one that will be important to potentially heavy users of an app like this–namely wedding photographers. An added benefit of this setup is the ability to load 35mm film in a medium format camera. You can do it in real life (with the right kind of film back) so why not in the app?