Fix issues reported by Flathub analysis

[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?

Flatpak has introduced various guidelines for what they consider to be required conditions for an app being of high quality. Rymdport manages to complete the vast majority bu there are areas where we can improve.

Guidelines: https://docs.flathub.org/docs/for-app-authors/appdata-guidelines/quality-guidelines/

However: I do not agree that they think our screenshots are out of date. Nothing has changes visually so why update them for the sake of it?

Describe the solution you'd like to see.

Address the following concerns:

• Add screenshots with the light theme on Gnome.
• Update all the screenshots for a small refresh.
• Update the icon to fill the canvas better.
• Update the icon to be in line with modern styles and have better contrast. Related to Redesign the application icon #97.

[Jacalz]

Looks like we also need to set brand colours:

• Update metainfo with brand colours: https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#has-primary-brand-colors

[Jacalz]

Most of this was solved for v3.5.3. I'll let the icon redesign happen later.