Add system-style macOS icon

[iLynxcat]

Description

This adds a proper system-style icon to fit in with macOS' icon style that's been adopted since Big Sur. In my original issue #322 I offered to include the original Sketch file for this but not sure where/if it fits in the repo or matters whatsoever to the project's maintainers.

Fixes #322

Type of change

This PR does not include any functionality change to the app and is purely cosmetic. Marking it as "new feature" because it's the best fit.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

This PR does not include any code so I'm checking these off anyway...

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[iLynxcat]

I have read the CLA Document and I hereby sign the CLA

[Neved4]

@iLynxcat This is cool! LGTM!

[stefanlight8]

Can you share .icns file?

[iLynxcat]

Can you share .icns file?

The .icns file is included in this PR

[iLynxcat]

Closing since there has been no activity from maintainers since opening — feel free to comment or reopen if this is considered