Add appdata file for the Fedora Software center

Solves #960

@Ablu Would such an appdata file be sufficient?

Anyway we should have our own quality screenshots in our wiki, not linking to tiled.
But to my understanding the appdata file makes the package visible to the enduser? And we don't want the enduser to know about the engine, but rather just add the dependencies to the games relying on the engine. So maybe we should just close this one and add it to all the games instead?