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/OS.cs

@@ -0,0 +1,119 @@
+using System;
+using System.Diagnostics;
+
+using Avalonia;
+
+namespace SourceGit.Native
+{
+    public static class OS
+    {
+        public interface IBackend
+        {
+            void SetupApp(AppBuilder builder);
+
+            string FindGitExecutable();
+            string FindVSCode();
+            string FindFleet();
+
+            void OpenTerminal(string workdir);
+            void OpenInFileManager(string path, bool select);
+            void OpenBrowser(string url);
+            void OpenWithDefaultEditor(string file);
+        }
+
+        public static string GitInstallPath { get; set; }
+
+        public static string VSCodeExecutableFile { get; set; }
+
+        public static string FleetExecutableFile { get; set; }
+
+        static OS()
+        {
+            if (OperatingSystem.IsWindows())
+            {
+                _backend = new Windows();
+            }
+            else if (OperatingSystem.IsMacOS())
+            {
+                _backend = new MacOS();
+            }
+            else if (OperatingSystem.IsLinux())
+            {
+                _backend = new Linux();
+            }
+            else
+            {
+                throw new Exception("Platform unsupported!!!");
+            }
+
+            VSCodeExecutableFile = _backend.FindVSCode();
+            FleetExecutableFile = _backend.FindFleet();
+        }
+
+        public static void SetupApp(AppBuilder builder)
+        {
+            _backend.SetupApp(builder);
+        }
+
+        public static string FindGitExecutable()
+        {
+            return _backend.FindGitExecutable();
+        }
+
+        public static void OpenInFileManager(string path, bool select = false)
+        {
+            _backend.OpenInFileManager(path, select);
+        }
+
+        public static void OpenBrowser(string url)
+        {
+            _backend.OpenBrowser(url);
+        }
+
+        public static void OpenTerminal(string workdir)
+        {
+            _backend.OpenTerminal(workdir);
+        }
+
+        public static void OpenWithDefaultEditor(string file)
+        {
+            _backend.OpenWithDefaultEditor(file);
+        }
+
+        public static void OpenInVSCode(string repo)
+        {
+            if (string.IsNullOrEmpty(VSCodeExecutableFile))
+            {
+                App.RaiseException(repo, "Visual Studio Code can NOT be found in your system!!!");
+                return;
+            }
+
+            Process.Start(new ProcessStartInfo()
+            {
+                WorkingDirectory = repo,
+                FileName = VSCodeExecutableFile,
+                Arguments = $"\"{repo}\"",
+                UseShellExecute = false,
+            });
+        }
+
+        public static void OpenInFleet(string repo)
+        {
+            if (string.IsNullOrEmpty(FleetExecutableFile))
+            {
+                App.RaiseException(repo, "Fleet can NOT be found in your system!!!");
+                return;
+            }
+
+            Process.Start(new ProcessStartInfo()
+            {
+                WorkingDirectory = repo,
+                FileName = FleetExecutableFile,
+                Arguments = $"\"{repo}\"",
+                UseShellExecute = false,
+            });
+        }
+
+        private static readonly IBackend _backend;
+    }
+}