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?
This looks like the way we should do it. We can have the preferences in several xml files and the preference handling in several PreferenceFragment classes. This should make it easier to read.  

I already tried refactoring the preferences like this but I have one problem:
I'm not able to put single preferences in the preference activity without beeing in a category. The push message checkbox for example doesn't really fit into a category, it would be good to display it above instead of creating an extra category with one option. 

The header file looks like this:

	<?xml version="1.0" encoding="utf-8"?>

	<preference-headers xmlns:android="http://schemas.android.com/apk/res/android">
		<header android:fragment="org.thoughtcrime.securesms.ApplicationPreferencesActivity$SettingsFragmentSmsMms"
				android:title="@string/preferences__sms_mms" />
		<header android:fragment="org.thoughtcrime.securesms.ApplicationPreferencesActivity$SettingsFragmentNotifications"
				android:title="@string/preferences__notifications" />
		<header android:fragment="org.thoughtcrime.securesms.ApplicationPreferencesActivity$SettingsFragmentAppProtection"
				android:title="@string/preferences__app_protection" />
		<header android:fragment="org.thoughtcrime.securesms.ApplicationPreferencesActivity$SettingsFragmentAppearance"
				android:title="@string/preferences__appearance" />
		<header android:fragment="org.thoughtcrime.securesms.ApplicationPreferencesActivity$SettingsFragmentStorage"
				android:title="@string/preferences__delete_old_messages" />

	</preference-headers>

But I cant add preferences there without a header.. You have an idea?