Create groundwork for additional settings

...chku/quasseldroid/ui/coresettings/CoreSettingsFragment.kt

app/src/main/res/drawable/ic_bell.xml

app/src/main/res/drawable/ic_settings.xml

app/src/main/res/layout/settings_list.xml

app/src/main/res/values-de/strings_settings.xml

app/src/main/res/values/strings_settings.xml