Revert the AppData file removal

[AsciiWolf]

Please, consider reverting this commit. AppStream metadata are a standard thing required by many distributions/package managers nowadays. Removing them makes no sense.

[AsciiWolf]

By the way, it looks like that the appdata file had a problem that prevented it to work with most AppStream parsers (except flatpak-builder) because they could not match the desktop file name. It should have contained this line:

<launchable type="desktop-id">pingus.desktop</launchable>

So, if the appdata file removal commit gets reverted, please also modify the file and add this line. Preferably also rename the appdata file from "pingus.appdata.xml.in" to "org.seul.pingus.Pingus.appdata.xml.in".