Update screenshots with default window borders

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Flathub metadata reviews did classify the screenshots as not having default settings, presumably because the window borders aren’t stock Gnome.

Describe the solution you'd like to see.

Update screenshots to either be taken with default buttons on Gnome or on Plasma instead.

[Jacalz]

Screenshots will have to be updated anyhow for v4.0.0 and UI redesign so lets schedule it there.