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/Archive.axaml.cs

@@ -0,0 +1,27 @@
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Platform.Storage;
+
+namespace SourceGit.Views
+{
+    public partial class Archive : UserControl
+    {
+        public Archive()
+        {
+            InitializeComponent();
+        }
+
+        private async void SelectOutputFile(object sender, RoutedEventArgs e)
+        {
+            var options = new FilePickerSaveOptions() { DefaultExtension = ".zip", FileTypeChoices = [new FilePickerFileType("ZIP") { Patterns = ["*.zip"] }] };
+            var toplevel = TopLevel.GetTopLevel(this);
+            var selected = await toplevel.StorageProvider.SaveFilePickerAsync(options);
+            if (selected != null)
+            {
+                txtSaveFile.Text = selected.Path.LocalPath;
+            }
+
+            e.Handled = true;
+        }
+    }
+}