After dataengine was ported away, it requires to use a self-maintained
engine to properly support multi-screen wallpapers.

3 new classes are added:

- PotdEngine: a class to manage clients, create and delete clients on
demand.
- Client: a class to process wallpaper data
- Backend: a class used in QML side, to relay information from the client

The major part of PotdProviderModel are moved to the 3 classes, so the
model class is now a pure model.

BUG: 454333
FIXED-IN: 5.26