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/Native/MacOS.cs

@@ -0,0 +1,86 @@
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.Runtime.Versioning;
+using System.Text;
+
+using Avalonia;
+using Avalonia.Media;
+
+namespace SourceGit.Native
+{
+    [SupportedOSPlatform("macOS")]
+    internal class MacOS : OS.IBackend
+    {
+        public void SetupApp(AppBuilder builder)
+        {
+            builder.With(new FontManagerOptions()
+            {
+                DefaultFamilyName = "PingFang SC",
+            });
+        }
+
+        public string FindGitExecutable()
+        {
+            if (File.Exists("/usr/bin/git"))
+                return "/usr/bin/git";
+            return string.Empty;
+        }
+
+        public string FindVSCode()
+        {
+            var toolPath = "/Applications/Visual Studio Code.app/Contents/Resources/app/bin/code";
+            if (File.Exists(toolPath))
+                return toolPath;
+            return string.Empty;
+        }
+
+        public string FindFleet()
+        {
+            var toolPath = $"{Environment.GetFolderPath(Environment.SpecialFolder.UserProfile)}/Applications/Fleet.app/Contents/MacOS/Fleet";
+            if (File.Exists(toolPath))
+                return toolPath;
+            return string.Empty;
+        }
+
+        public void OpenBrowser(string url)
+        {
+            Process.Start("open", url);
+        }
+
+        public void OpenInFileManager(string path, bool select)
+        {
+            if (Directory.Exists(path))
+            {
+                Process.Start("open", path);
+            }
+            else if (File.Exists(path))
+            {
+                Process.Start("open", $"\"{path}\" -R");
+            }
+        }
+
+        public void OpenTerminal(string workdir)
+        {
+            var dir = string.IsNullOrEmpty(workdir) ? "~" : workdir;
+            var builder = new StringBuilder();
+            builder.AppendLine("on run argv");
+            builder.AppendLine("    tell application \"Terminal\"");
+            builder.AppendLine($"        do script \"cd '{dir}'\"");
+            builder.AppendLine("        activate");
+            builder.AppendLine("    end tell");
+            builder.AppendLine("end run");
+
+            var tmp = Path.GetTempFileName();
+            File.WriteAllText(tmp, builder.ToString());
+
+            var proc = Process.Start("/usr/bin/osascript", $"\"{tmp}\"");
+            proc.Exited += (o, e) => File.Delete(tmp);
+        }
+
+        public void OpenWithDefaultEditor(string file)
+        {
+            Process.Start("open", file);
+        }
+    }
+}