Update AppData file

[AsciiWolf]

Add launchable tag that is now required by many AppStream parsers.

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/cc @Benau

Also consider renaming the AppData file (and the app id) to a rDNS (net.supertuxkart.SuperTuxKart) format. The non-rDNS one is deprecated.

[AsciiWolf]

@CodingJellyfish @Alayan-stk-2 Could this please be merged? Thanks!

[CodingJellyfish]

I'm so sorry, but I'm not very familiar with appdata file atm. So I couldn't make any decision with it.

[AsciiWolf]

Here is an official documentation for AppStream: https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Application.html#tag-dapp-launchable

"It is required that a launchable tag is present in desktop-application MetaInfo files, unless the MetaInfo data itself contains all required information that a desktop-entry file would have (categories, icon data, ...) and the application can not be launched standalone. This makes the launchable tag a required tag for pretty much all desktop-application components, with only very rare exceptions."

We have to patch this on Flathub: https://github.com/flathub/net.supertuxkart.SuperTuxKart/blob/master/add-app-data-launchable.patch

[AsciiWolf]

It basically just tells the AppStream parser what the name of the SuperTuxKart desktop file is - it is useful if the AppStream metadata file id does not match the actual desktop file name, but it does no harm even if the metadata id matches the desktop name, and as I pointed out, it is becoming required by most AppStream parsers nowadays.

[AsciiWolf]

@Benau Maybe you could provide some feedback here? :-)

[AsciiWolf]

@CodingJellyfish Looks like you are the only active STK team member at the moment. Any chance you could take a look at the official AppStream documentation link (and also the Flathub downstream patch) here and decide whether to merge this PR or not? Thanks!

[CodingJellyfish]

If any regression is created, feel free to let us know.