Distribute the macOS .app directly

[orazioedoardo]

The macOS version of the manager is distributed as a package installer, which is effectively just a wrapper for a simple operation that copies the .app to /Applications. This requires admin privileges and is not necessary, as the app works from any location (as it should) and unprivileged users may want to copy to ~/Applications instead. Recommend just bundling YubiKey Manager.app in a dmg with the usual convention of the app icon arranged with an arrow pointing to a link to /Applications next to it.