Patch 1/2 (other patch is for plasma-desktop)
BUG: #386621
* https://bugs.kde.org/show_bug.cgi?id=386621
* To find the `Categories=Plasma Wallpaper Plugin` in the `wallpaperplugin.knsrc`, I had to find the `<name>Plasma Wallpaper Plugin</name>` on the this API url: https://api.kde-look.org/ocs/v1/content/categories
* To find that url, I looked at: https://github.com/KDE/ocs-webserver/blob/master/application/modules/default/Bootstrap.php#L1014
Screenshot of the dialog:
{F5800268}
This screenshot is more releavant to Patch 2 for plasma-desktop:
{F5800267}
## TODO:
* [ ] Move C++ plugin to `components/shellprivate/wallpaperplugin` ? I probably need to move the new GHNS code to `components/shellprivate/wallpaperplugin` since "desktoppackage" has the widgetexplorer C++ plugins there. I figured I'd put up a working patch first.
* [ ] Update `wallpaperComboBox` and `configDialog.wallpaperConfigModel` so we don't have to close and reopen the "Configure Desktop" window for the plugin to show up in the dropdown.