Put the macOS icon in a rounded rectangle

[maksalees]

The current icon for macOS does not follow design guidelines introduced in Big Sur and therefore looks out of place. I placed the icon in a rounded rectangle to make it look like the rest of macOS icons.

Closes #3946

Before

After

Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[maksalees]

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