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.
Hmm, we don't provide `flare-engine.desktop` any more since it doesn't make sense to run the engine by itself. Instead, each "game" provides its own desktop file that has an Exec line like:
```
flare --game=flare-game
```

Not sure what the right solution here is. Do we re-add a desktop file to this repo?