Add Avalonia based desktop client

Squashed commit of the following:

commit 0bfba90cd31bc9d401bf25cd479f6aef314988d5
Author: luxick <git@luxick.de>
Date:   Mon Mar 2 21:58:32 2020 +0100

    Add view switching with Global.Navigator

commit e0f535152aaa31fc2d4b6712459d208e27831696
Author: luxick <git@luxick.de>
Date:   Mon Mar 2 20:53:01 2020 +0100

    Make ViewModels require api client

commit f66c9148fdb1c34a2b92321dfc7a053894f381ce
Author: luxick <git@luxick.de>
Date:   Mon Mar 2 20:51:35 2020 +0100

    Update framework to core 3.1

commit 273b2940f043d889c70e8d7a69a54d6b4b200a62
Author: luxick <git@luxick.de>
Date:   Mon Mar 2 20:49:43 2020 +0100

    Player and Season View/Models

commit b3325b176740c403381009882c98a401ece05d0a
Author: luxick <git@luxick.de>
Date:   Mon Mar 2 20:44:55 2020 +0100

    Add EstusShots.Avalonia project

EstusShots.Avalonia/Program.cs

@@ -0,0 +1,24 @@
+using System;
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Logging.Serilog;
+using Avalonia.ReactiveUI;
+
+namespace EstusShots.Avalonia
+{
+    class Program
+    {
+        // Initialization code. Don't use any Avalonia, third-party APIs or any
+        // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
+        // yet and stuff might break.
+        public static void Main(string[] args) => BuildAvaloniaApp()
+            .StartWithClassicDesktopLifetime(args);
+
+        // Avalonia configuration, don't remove; also used by visual designer.
+        public static AppBuilder BuildAvaloniaApp()
+            => AppBuilder.Configure<App>()
+                .UsePlatformDetect()
+                .LogToDebug()
+                .UseReactiveUI();
+    }
+}