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.
- 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: 5.25.1

(cherry picked from commit 86ef83c6)

wallpapers/potd: increase m_updateCount before `updateSource`

When cache is available, `PotdClient::done` will be directly emitted.

(cherry picked from commit 9a1180f1)

wallpapers/potd: remove redundant if statement

The while loop already checks the condition.

(cherry picked from commit 6633fa60)

wallpapers/potd: add some debug information

(cherry picked from commit 09c8337f)