Icon is missing in KDE

When using Cockpit Client in KDE, the icon is missing. I think this is a problem with the Flatpakization, as the icon does show up in Flathub and also in GNOME just fine. Additionally, the icon in the launcher is poor quality and is missing transparency.

So we might be missing a line here in the YML @ https://github.com/flathub/org.cockpit_project.CockpitClient/blob/master/org.cockpit_project.CockpitClient.yml

Perhaps we need a special install line similar to the metainfo, but for the SVG?
https://github.com/flathub/org.cockpit_project.CockpitClient/blob/9bb830c1953817668ddf277ddfb29acaff34f966/org.cockpit_project.CockpitClient.yml#L29-L30

Something like this, perhaps:
```yaml
- install -D -t ${FLATPAK_DEST}/share/icons/hicolor/scalable/apps org.cockpit_project.CockpitClient.svg
- install -D -t ${FLATPAK_DEST}/share/icons/hicolor/128x128/apps org.cockpit_project.CockpitClient.png
```

(It should be the case that only an SVG is needed, but when I was flatpaking Lunacy, the builder complained about not having a 128px PNG, so I included an export of the SVG.)