Convert metadata.desktop to metadata.json for wallpapers

[Pseudo-Account]

According to the KDE Documentation:

Plasmoids previously used a metadata.desktop file.
This is discouraged, because the conversion to JSON will need to be done at runtime.
Shipping a JSON file directly is supported for all of Plasma 5.

So, that commit won't break compatibility with the current version of NX Desktop.

Also, I had tried to install wallpaper pack from that theme on my PC (KDE neon 6.0, KDE Plasma Version: 6.0.5), but it did't appear in Settings menu.
Only after converting files with desktoptojson utility all the pictures became available.

Best regards,
Andrey

P.S. The same issue is confirmed with the wallpapers in related packages: Nitrux/nitrux-wallpapers-free and nx-desktop/nx-plasma-look-and-feel