SettingsAccessor: Shed of UserFileAccessor

So far only instantiates the .user file specific upgraders.

Change-Id: I9a16dfe4c3c61093ea1231413495eec2e8fe648e
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>