wallpapers/potd: fix multimonitor support
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. It will be automatically deleted when there is no Backend.
- 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 class is a pure model now.
BUG: 454333 FIXED-IN: 5.26