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?
done, altought I'm not 100% sure about the naming. How did you get the names of gray5, gray95 etc? I used `100 - Lightness / Value` from the HSL/HSV color models.