SettingsAccessor: Inline writeFile, it's only called once

Change-Id: Iba22b820ce5b671a924757d5a24900e20f8c46ae
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Move SettingsAccessor::SettingsData to private class

Also rename it into Settings.

Change-Id: Id01f664b0cac4de58ffe52eea1aeeaf9cf0891d5
Reviewed-by: Daniel Teske <daniel.teske@digia.com>