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/ViewModels/MainWindowViewModel.cs

@@ -0,0 +1,25 @@
+using System;
+using ReactiveUI;
+
+namespace EstusShots.Avalonia.ViewModels
+{
+    public class MainWindowViewModel : ViewModelBase
+    {
+        public IServiceProvider Container;
+        
+        private ViewModelBase _content;
+
+        public MainWindowViewModel(IServiceProvider container)
+        {
+            Container = container;
+        }
+
+        public ViewModelBase Content    
+        {
+            get => _content;
+            set => this.RaiseAndSetIfChanged(ref _content, value);
+        }
+
+        
+    }
+}

EstusShots.Avalonia/ViewModels/PlayersViewModel.cs

@@ -0,0 +1,14 @@
+using EstusShots.Client;
+
+namespace EstusShots.Avalonia.ViewModels
+{
+    public class PlayersViewModel : ViewModelBase
+    {
+        private readonly EstusShotsClient _apiClient;
+
+        public PlayersViewModel(EstusShotsClient apiClient)
+        {
+            _apiClient = apiClient;
+        }
+    }
+}

EstusShots.Avalonia/ViewModels/SeasonsViewModel.cs

@@ -0,0 +1,26 @@
+using System.Collections.Generic;
+using System.Reactive;
+using EstusShots.Client;
+using EstusShots.Shared.Dto;
+using ReactiveUI;
+
+namespace EstusShots.Avalonia.ViewModels
+{
+    public class SeasonsViewModel : ViewModelBase
+    {
+        private readonly EstusShotsClient _apiClient;
+        private readonly MainWindowViewModel _main;
+
+        public List<Season> Seasons { get; set; }
+
+        public ReactiveCommand<Unit, Unit> ToPlayers { get; }
+        
+        public SeasonsViewModel(EstusShotsClient apiClient, MainWindowViewModel main)
+        {
+            _apiClient = apiClient;
+            _main = main;
+
+            ToPlayers = ReactiveCommand.Create(() => { Global.Navigator.GoTo<PlayersViewModel>(); });
+        }
+    }
+}

EstusShots.Avalonia/ViewModels/ViewModelBase.cs

@@ -0,0 +1,11 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using ReactiveUI;
+
+namespace EstusShots.Avalonia.ViewModels
+{
+    public class ViewModelBase : ReactiveObject
+    {
+    }
+}