Which calls gtk4-update-icon-cache if available.
Fixes the build when gtk3 is not in builder sandbox.

- Add vcs-browser URL
- Update donation URL
- Add missing launchable tag

"The old generator also included all the none-value entries—it
no longer does this, you can safely remove them.

If it only consists of none values, it’s safe to shorten it to just:
`<content_rating type="oars-1.1" />`"

More information: https://docs.flathub.org/docs/for-app-authors/appdata-guidelines/#oars-information