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/Models/Decorator.cs

@@ -0,0 +1,27 @@
+using Avalonia.Media;
+
+namespace SourceGit.Models
+{
+    public enum DecoratorType
+    {
+        None,
+        CurrentBranchHead,
+        LocalBranchHead,
+        RemoteBranchHead,
+        Tag,
+    }
+
+    public class Decorator
+    {
+        public DecoratorType Type { get; set; } = DecoratorType.None;
+        public string Name { get; set; } = "";
+    }
+
+    public static class DecoratorResources
+    {
+        public static readonly IBrush[] Backgrounds = [
+            new SolidColorBrush(0xFF02C302),
+            new SolidColorBrush(0xFFFFB835),
+        ];
+    }
+}