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/Linux.cs

@@ -0,0 +1,123 @@
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.Runtime.Versioning;
+
+using Avalonia;
+using Avalonia.Dialogs;
+using Avalonia.Media;
+
+namespace SourceGit.Native
+{
+    [SupportedOSPlatform("linux")]
+    internal class Linux : OS.IBackend
+    {
+        public void SetupApp(AppBuilder builder)
+        {
+            builder.With(new FontManagerOptions()
+            {
+                DefaultFamilyName = "fonts:SourceGit#JetBrains Mono",
+            });
+
+            // Free-desktop file picker has an extra black background panel.
+            builder.UseManagedSystemDialogs();
+        }
+
+        public string FindGitExecutable()
+        {
+            if (File.Exists("/usr/bin/git"))
+                return "/usr/bin/git";
+            return string.Empty;
+        }
+
+        public string FindVSCode()
+        {
+            var toolPath = "/usr/share/code/code";
+            if (File.Exists(toolPath))
+                return toolPath;
+            return string.Empty;
+        }
+
+        public string FindFleet()
+        {
+            var toolPath = $"{Environment.GetFolderPath(Environment.SpecialFolder.UserProfile)}/.local/share/JetBrains/Toolbox/apps/fleet/bin/Fleet";
+            if (File.Exists(toolPath))
+                return toolPath;
+            return string.Empty;
+        }
+
+        public void OpenBrowser(string url)
+        {
+            if (!File.Exists("/usr/bin/xdg-open"))
+            {
+                App.RaiseException("", $"You should install xdg-open first!");
+                return;
+            }
+
+            Process.Start("xdg-open", $"\"{url}\"");
+        }
+
+        public void OpenInFileManager(string path, bool select)
+        {
+            if (!File.Exists("/usr/bin/xdg-open"))
+            {
+                App.RaiseException("", $"You should install xdg-open first!");
+                return;
+            }
+
+            if (Directory.Exists(path))
+            {
+                Process.Start("xdg-open", $"\"{path}\"");
+            }
+            else
+            {
+                var dir = Path.GetDirectoryName(path);
+                if (Directory.Exists(dir))
+                {
+                    Process.Start("xdg-open", $"\"{dir}\"");
+                }
+            }
+        }
+
+        public void OpenTerminal(string workdir)
+        {
+            var dir = string.IsNullOrEmpty(workdir) ? "~" : workdir;
+            if (File.Exists("/usr/bin/gnome-terminal"))
+            {
+                Process.Start("/usr/bin/gnome-terminal", $"--working-directory=\"{dir}\"");
+            }
+            else if (File.Exists("/usr/bin/konsole"))
+            {
+                Process.Start("/usr/bin/konsole", $"--workdir \"{dir}\"");
+            }
+            else if (File.Exists("/usr/bin/xfce4-terminal"))
+            {
+                Process.Start("/usr/bin/xfce4-terminal", $"--working-directory=\"{dir}\"");
+            }
+            else
+            {
+                App.RaiseException("", $"Only supports gnome-terminal/konsole/xfce4-terminal!");
+                return;
+            }
+        }
+
+        public void OpenWithDefaultEditor(string file)
+        {
+            if (!File.Exists("/usr/bin/xdg-open"))
+            {
+                App.RaiseException("", $"You should install xdg-open first!");
+                return;
+            }
+
+            var proc = Process.Start("xdg-open", $"\"{file}\"");
+            proc.WaitForExit();
+
+            if (proc.ExitCode != 0)
+            {
+                App.RaiseException("", $"Failed to open \"{file}\"");
+            }
+
+            proc.Close();
+        }
+    }
+}