Redesign the application icon

[Jacalz]

Checklist

• [X] I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• [X] 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?

The current icon has suited us surprisingly well over the years but I have long thought that we should try to get an icon that looks better. I designed the icon in Inkscape many years ago now without any real knowledge of design. Someone better versed in design should be able to come up with something better looking :)

Describe the solution you'd like to see.

A new icon that looks modern and stylish while keeping with the space/wormhole ideas.

[Jacalz]

We might want to set up app brand colours also: https://docs.flathub.org/blog/introducing-app-brand-colors/