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/Converters/WindowStateConverters.cs

@@ -0,0 +1,58 @@
+using System;
+
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Data.Converters;
+using Avalonia.Media;
+
+namespace SourceGit.Converters
+{
+    public static class WindowStateConverters
+    {
+        public static readonly FuncValueConverter<WindowState, Thickness> ToContentMargin =
+            new FuncValueConverter<WindowState, Thickness>(state =>
+            {
+                if (OperatingSystem.IsWindows() && state == WindowState.Maximized)
+                {
+                    return new Thickness(6);
+                }
+                else if (OperatingSystem.IsLinux() && state != WindowState.Maximized)
+                {
+                    return new Thickness(6);
+                }
+                else
+                {
+                    return new Thickness(0);
+                }
+            });
+
+        public static readonly FuncValueConverter<WindowState, GridLength> ToTitleBarHeight =
+            new FuncValueConverter<WindowState, GridLength>(state =>
+            {
+                if (state == WindowState.Maximized)
+                {
+                    return new GridLength(30);
+                }
+                else
+                {
+                    return new GridLength(38);
+                }
+            });
+
+        public static readonly FuncValueConverter<WindowState, StreamGeometry> ToMaxOrRestoreIcon =
+            new FuncValueConverter<WindowState, StreamGeometry>(state =>
+            {
+                if (state == WindowState.Maximized)
+                {
+                    return Application.Current?.FindResource("Icons.Window.Restore") as StreamGeometry;
+                }
+                else
+                {
+                    return Application.Current?.FindResource("Icons.Window.Maximize") as StreamGeometry;
+                }
+            });
+
+        public static readonly FuncValueConverter<WindowState, bool> IsNormal =
+            new FuncValueConverter<WindowState, bool>(state => state == WindowState.Normal);
+    }
+}