Plasma 6 support

[principis]

As of Plasma 6 all themes are now handled as kpackage, which means that a manifest.json file is required.

This PR ports the metadata.desktop files to metadata.json. This supports both Plasma 5 and Plasma 6.

FYI: this PR is backported in Fedora 40.

[principis]

The switch.svg widget is broken in Plasma 6, and I don't know how to fix it, so I removed it. It falls back to the default widget which looks fine.

[ItachiSan]

There are a few more metadata.desktop files, do they not need conversion too?
Was just experimenting converting them locally with desktoptojson

[principis]

I don't think so, at least not for plasma theming. Please point me to the relevant documentation (if it exists...), and I'll be happy to migrate them.

A plain .desktop to .json is not recommended, as there are the packaging format changes, not just the file format.

[principis]

Converted the wallpapers as well. Otherwise they wouldn't show up in the wallpaper view.

I believe @ItachiSan was right, and everything needs to be converted. There are still a few .desktop files left, but I haven't encountered them yet. If anyone knows where the issue occurs, please let me know where so that I can port and test it!