Add AppStream Metadata for KSysGuard, since it's currently lacking.
Nitpick: Use desktop-application
Maybe using FSFAP as license is better here, so the sources don't have to ship a copy of the CC0 license.
Thumbnails are generally thrown away be the metadata extractor on the Linux distro side, so you can really drop the "type" property of the image tag here and just add the source image.
The component is missing a launchable tag: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-launchable
FWIW, @mak, appstrealcli validate doesn't like my launchable tag, though I can't see anything wrong with it.
dev@dev-pc:~/repos/ksysguard$ (appstream-metadata) appstreamcli validate gui/org.kde.ksysguard.appdata.xml W - org.kde.ksysguard.appdata.xml:org.kde.ksysguard:7 Found invalid tag: 'launchable'. Non-standard tags must be prefixed with "x-". Validation failed: warnings: 1