Please provide a better screenshot

The appdata points to https://29a.ch/playitslowly/screenshot.png

That screenshot is quite small, which fails the appstream validation:

```
❯ appstream-util validate share/appdata/ch.x29a.playitslowly.appdata.xml
share/appdata/ch.x29a.playitslowly.appdata.xml: FAILED:
• attribute-invalid : width (552) too small [https://29a.ch/playitslowly/screenshot.png] minimum is 624px
• attribute-invalid : height too small [https://29a.ch/playitslowly/screenshot.png] minimum is 351px
Validation of files failed
```

It's not just a bureaucratic thing though; on HiDPI screens the screenshot will look extremely small and bad, making Play it Slowly look bad.

I've just taken https://www.daitauha.fr/static/playitslowly/screenshot.png which should look better on HiDPI screens (as well as regular ones of course) and passes the validation.

Feel free to use it to replace the current one if it works for you. :slightly_smiling_face:

(my screenshot uses the default GNOME style and wallpaper, because that's what I use here; I hope that's not a problem, but that's not the point anyway: the point is to have a reasonably-sized screenshot)

Update share/appdata/ch.x29a.playitslowly.appdata.xml, replacing the current screenshot URL with the proposed larger image at https://www.daitauha.fr/static/playitslowly/screenshot.png. Run appstream-util validate share/appdata/ch.x29a.playitslowly.appdata.xml; done means validation passes with screenshot dimensions meeting the minimum.

Written by the indexing model from the issue text.