preference improvements

- Configuration inherits from PreferenceActivity
- generic PreferenceFragmentLoader implemented (renamed ConfigurationFragment)
- backStack support implemented
- PreferenceSwitchable implemented (enables switchable PreferenceScreens. see preferences.xml)
- separation of preferences for different application parts (preferences.xml, preferences_phone.xml)
- preview for phone settings (a simple AlertDialog by now; configurable via xml see. preferences_phone.xml)
- debug on/off via about preference (7 times tapping to switch)