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/Locales.cs

@@ -0,0 +1,21 @@
+using System.Collections.Generic;
+
+namespace SourceGit.Models
+{
+    public class Locale
+    {
+        public string Name { get; set; }
+        public string Key { get; set; }
+
+        public static readonly List<Locale> Supported = new List<Locale>() {
+            new Locale("English", "en_US"),
+            new Locale("简体中文", "zh_CN"),
+        };
+
+        public Locale(string name, string key)
+        {
+            Name = name;
+            Key = key;
+        }
+    }
+}