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 logging category `kde.wallpapers.image` for PoTD plugin.

Task: https://phabricator.kde.org/T13323