[Feature Request] Use included screenshot

I think the best UX is to use the screenshot specified in appdata.metainfo. I even wrote a small parser (I managed to get the right result, but I'm sure it won't always be like this)

```bash
cat $METAFILE | grep '' | sed "s/^\ *<[^>]*>//g;s/<\/[^>]*>//g" | cut -d'>' -f2 # For 1 screenshot
cat $METAFILE | grep '' | sed "s/^\ *<[^>]*>//g;s/<\/[^>]*>//g" | cut -d '>' -f1 | head -n1 # For multiple screenshots
```

Benefits:

* User see app in better resolution
* this will simplify worker.sh

Warning:

Not all include appdata.xml or metainfo.xml in your AppImage