added SwitchablePreferenceScreen

the SwitchablePreferenceScreen is inspired by the Android Settings Application on Android 4+. It is simply a SwitchPreference which you can press and open just like a PreferenceScreen, as you can do it with the WiFi, Bluetooth Preferences on Android.

I added indicators to it because it is not obvious that you can also open the SwitchPreference as a PreferenceScreen :)