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/Apply.axaml.cs

@@ -0,0 +1,30 @@
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Platform.Storage;
+
+namespace SourceGit.Views
+{
+    public partial class Apply : UserControl
+    {
+        public Apply()
+        {
+            InitializeComponent();
+        }
+
+        private async void SelectPatchFile(object sender, RoutedEventArgs e)
+        {
+            var topLevel = TopLevel.GetTopLevel(this);
+            if (topLevel == null)
+                return;
+
+            var options = new FilePickerOpenOptions() { AllowMultiple = false, FileTypeFilter = [new FilePickerFileType("Patch File") { Patterns = ["*.patch"] }] };
+            var selected = await topLevel.StorageProvider.OpenFilePickerAsync(options);
+            if (selected.Count == 1)
+            {
+                txtPatchFile.Text = selected[0].Path.LocalPath;
+            }
+
+            e.Handled = true;
+        }
+    }
+}