non-Cairo/X11 lstopo GUI on Mac OS

[bgoglin]

We often get complaints from MacOS users because they install Cairo+hwloc from homebrew but they don't get a X11-enable lstopo. Then they rebuild hwloc from source, which says it has Cairo, but still no X11 lstopo. The reason is that homebrew disables X11 in Cairo because X11 isn't available by default on MacOS. And it looks like it won't change anytime soon (ref Homebrew/homebrew-core#27462).

So they would rather see us to add another GUI backend than relying on X11 on MacOS, for instance using https://developer.apple.com/swift/