Add a Big Sur-style rounded rectangle icon on macOS

[fwcd]

Fixes https://bugs.launchpad.net/mixxx/+bug/1952532.

This PR adds a macOS Big Sur-style icon for macOS, following the conventions outlined in Apple's Human Interface Guidelines:

With this icon, Mixxx looks more 'at home' with other macOS apps:

The new icon is provided both as an SVG and as corresponding ICNS files.

[ronso0]

Thank you!
The Human Interface Guidelines doesn't mention SVG, though it works, and it explicitely states "Don’t provide app icons in ICNS or JPEG format."
Can you elaborate?

[ronso0]

A few issues regarding hidden objects.
This is nit-picking, I know, but when working with image ressources I often noticed we drag 'dead freight' along, so let's clean that up if we add a new file.

[fwcd]

I chose SVG as a source format to have a scalable vector version of the icon, just like the existing one. The SVG version is only for development though, the icon that is actually used for the app bundle is a rendered/rasterized version, i.e. the ICNS files.

The motivation for using ICNS for now is that the existing icon uses it, thereby making it a drop-in replacement for now. Updating to an asset bundle would be something I'd rather put in a separate PR since it might require changes to the build/packaging process on macOS.

[ronso0]

Thanks.
Btw I think when you're done with the clean-up you can squash your commits into one update macOS app icon, rounded rectangle

[fwcd]

The branch should now be rebased, squashed and ready for merge.

[ronso0]

I couldn't extract all PNG from the icns, but @foss- helped verifying those.

LGTM Thank you @fwcd!