project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/Views/About.axaml.cs

@@ -0,0 +1,59 @@
+using System.Reflection;
+
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+
+namespace SourceGit.Views
+{
+    public partial class About : Window
+    {
+        public string Version
+        {
+            get;
+            private set;
+        }
+
+        public About()
+        {
+            var ver = Assembly.GetExecutingAssembly().GetName().Version;
+            Version = $"{ver.Major}.{ver.Minor}";
+            DataContext = this;
+            InitializeComponent();
+        }
+
+        private void BeginMoveWindow(object sender, PointerPressedEventArgs e)
+        {
+            BeginMoveDrag(e);
+        }
+
+        private void CloseWindow(object sender, RoutedEventArgs e)
+        {
+            Close();
+        }
+
+        private void OnVisitAvaloniaUI(object sender, PointerPressedEventArgs e)
+        {
+            Native.OS.OpenBrowser("https://www.avaloniaui.net/");
+            e.Handled = true;
+        }
+
+        private void OnVisitAvaloniaEdit(object sender, PointerPressedEventArgs e)
+        {
+            Native.OS.OpenBrowser("https://github.com/AvaloniaUI/AvaloniaEdit");
+            e.Handled = true;
+        }
+
+        private void OnVisitJetBrainsMonoFont(object sender, PointerPressedEventArgs e)
+        {
+            Native.OS.OpenBrowser("https://www.jetbrains.com/lp/mono/");
+            e.Handled = true;
+        }
+
+        private void OnVisitSourceCode(object sender, PointerPressedEventArgs e)
+        {
+            Native.OS.OpenBrowser("https://github.com/sourcegit-scm/sourcegit");
+            e.Handled = true;
+        }
+    }
+}