Regroup preferences

I regrouped the preferences like discussed in the mailing list. I put notifications, old message deletion and passphrase into their own category pages. This is saving us ten items in the preference list. Its looking like this now:

![settings_red](https://cloud.githubusercontent.com/assets/5296073/3365453/6671a82a-fb2e-11e3-9837-ecf3829fbbc2.png)
![notifications](https://cloud.githubusercontent.com/assets/5296073/3365455/682daf7e-fb2e-11e3-90bf-fd5d964112e0.png)

But I dont know how to add the "back arrow" in the red square to the new PreferenceScreen's. Do you know how to put it there?

And I'm not sure about the new category for the PreferenceScreen's. I put them in "Miscellaneous" now, but perhaps you have a better idea?
since it looks like `fragment` should never be null, might make more sense to throw an `AssertionError` or at least log a warning in this exception case. If you use a switch, this can just be done in the `default` section.