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/SelfUpdate.axaml.cs

@@ -0,0 +1,39 @@
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+
+namespace SourceGit.Views
+{
+    public partial class SelfUpdate : Window
+    {
+        public SelfUpdate()
+        {
+            InitializeComponent();
+        }
+
+        private void BeginMoveWindow(object sender, PointerPressedEventArgs e)
+        {
+            BeginMoveDrag(e);
+        }
+
+        private void CloseWindow(object sender, RoutedEventArgs e)
+        {
+            Close();
+        }
+
+        private void GotoDownload(object sender, RoutedEventArgs e)
+        {
+            Native.OS.OpenBrowser("https://github.com/sourcegit-scm/sourcegit/releases/latest");
+            e.Handled = true;
+        }
+
+        private void IgnoreThisVersion(object sender, RoutedEventArgs e)
+        {
+            var button = sender as Button;
+            var ver = button.DataContext as Models.Version;
+            ViewModels.Preference.Instance.IgnoreUpdateTag = ver.TagName;
+            Close();
+            e.Handled = true;
+        }
+    }
+}