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/FileHistories.axaml.cs

@@ -0,0 +1,42 @@
+using Avalonia.Controls;
+using Avalonia.Input;
+
+namespace SourceGit.Views
+{
+    public partial class FileHistories : Window
+    {
+        public FileHistories()
+        {
+            InitializeComponent();
+        }
+
+        private void MaximizeOrRestoreWindow(object sender, TappedEventArgs e)
+        {
+            if (WindowState == WindowState.Maximized)
+            {
+                WindowState = WindowState.Normal;
+            }
+            else
+            {
+                WindowState = WindowState.Maximized;
+            }
+            e.Handled = true;
+        }
+
+        private void CustomResizeWindow(object sender, PointerPressedEventArgs e)
+        {
+            if (sender is Border border)
+            {
+                if (border.Tag is WindowEdge edge)
+                {
+                    BeginResizeDrag(edge, e);
+                }
+            }
+        }
+
+        private void BeginMoveWindow(object sender, PointerPressedEventArgs e)
+        {
+            BeginMoveDrag(e);
+        }
+    }
+}