Initial support for external/multiple networks in vmnet host mode

[Schamper]

This adds support for connecting QEMU VMs to specific host networks using the net-uuid option (described here). This can be extremely useful when connecting multiple VMs together, or for connecting VMs across virtualisation applications (VMware Fusion/Parallels). Unfortunately, I didn't find such option for the Apple virtualisation APIs, so it's only supported for QEMU VMs. #4190 essentially accomplished this by manually editing the QEMU arguments.

Note that using a custom network UUID will disable the DHCP that macOS vmnet would otherwise provide. As of right now, the user is responsible for providing their own DHCP on the network. E.g. by running a VM that provides it, or running a VM in e.g. VMware on the same network.

I've also added a button for importing a VMware Fusion networking configuration. It requires you to browse to and select /Library/Preferences/VMware Fusion/networking. All defined networks in there will be added. As mentioned before, I'm not aware of a more idiomatic approach for this. Adding support for other virtualisation applications is possible, but Fusion is my personal daily driver so that's what I know and have.

This is my first time writing any Swift code, so please excuse the code quality. I only started reading the basics of Swift a few days ago, so I'm not aware of any of the idiomatic or recommended ways of achieving certain things. I'll take any suggestions on that front.