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?
I really like the lighter summary colors, just don't want to have negative side-effects to the readability of other parts of the app :). Not sure if you can *just* set it for this activity for example, or what. There must be a way (although I've said that before and Android has surprised me).