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/BranchConverters.cs

@@ -0,0 +1,10 @@
+using Avalonia.Data.Converters;
+
+namespace SourceGit.Converters
+{
+    public static class BranchConverters
+    {
+        public static readonly FuncValueConverter<Models.Branch, string> ToName =
+            new FuncValueConverter<Models.Branch, string>(v => v.IsLocal ? v.Name : $"{v.Remote}/{v.Name}");
+    }
+}