Design the main icon

[matrohin]

We need an icon for the app:

• for the window (embedded into the binary).
• for the desktop entry.

I'm not a designer, so this one will get postponed for now, but I decided to create an issue to invite external contribution if anyone wants to pick this up.

[matrohin]

If you do want to pick this up, then please first share what you have in mind here so that we can discuss it.