feat(Avalonia): Bump Avalonia to 12 and a ton of other things.

[CaasGit]

A few things to test:

1. Windows build, does it work? Can you connect to servers still?
2. Mac build, same as above.
3. Does SetUnixFileMode work as intended within ChmodPlusX? I believe it's used to download CEF?
4. Worth looking at bumping nunit and it's friends, I didn't touch them since I just wanted to focus on the rest but they are out of date a bit.
5. There's a number of warnings about Avalonia's obsolete functions and Reactive source generation. There's also a older PR to remove most of Reactive that may be able to be ported over which would possibly fix one part of this, but it itself needs testing with Avalonia 12/this branch.
6. Test the drag and drop functions for the launcher, Avalonia reworked these and I poked it a bit, but it needs a good test, most of that is in SS14.Launcher/Views/MainWindow.xaml.cs.
7. The nix import file will need updating, I don't know or have any nix setup. We'll need someone who has experience with this.