add developer_name to appstream data file

[anarcat]

It seems this is necessary in Flatpaks now. I have a build that failed with:

flatpak run --command=flatpak-builder-lint org.flatpak.Builder --exceptions repo repo
 in dir /srv/buildbot/worker/build-x86_64-1/build (timeout 1200 secs)
 watching logfiles {}
 argv: b'flatpak run --command=flatpak-builder-lint org.flatpak.Builder --exceptions repo repo'
 using PTY: False
{
    "errors": [
        "appstream-missing-developer-name"
    ],
    "warnings": [
        "appstream-summary-too-long",
        "appstream-screenshot-missing-caption"
    ]
}
Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter
program finished with exit code 1
elapsedTime=54.274905

... interestingly, that is marked as "warning" in the above documentation:

https://docs.flathub.org/docs/for-app-authors/linter/#appstream-missing-developer-name

... but is reported as an error now. Even worse, the tag is deprecated:

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer_name

I tried to fix it by vendoring this fixed version in the flatpak, we'll see how that goes:

flathub/io.github.dweymouth.supersonic#62 (comment)

[anarcat]

don't merge until we get confirmation this is fixed in the flatpak, in flathub/io.github.dweymouth.supersonic#62 (comment)

[anarcat]

looks like that passed, so this is ready to go. filed flathub-infra/flatpak-builder-lint#273 which has some explanations on the change.

[anarcat]

@dweymouth i think this can be merged, at your convenience. of course you might want to tweak the name as you see fit...