wallpapers/potd: Port away from dataengines
Add PotdProvidersModel to provide a list of PoTD providers for the
wallpaper settings.
Drop m_canDiscardCache because it's only set to true in sourceRequestEvent,
which is gone with dataengines.
Add PotdProvidersModel to provide a list of PoTD providers for the
wallpaper settings.
Drop m_canDiscardCache because it's only set to true in sourceRequestEvent,
which is gone with dataengines.