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/ViewModels/AddSubmodule.cs

@@ -0,0 +1,76 @@
+using System.ComponentModel.DataAnnotations;
+using System.IO;
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class AddSubmodule : Popup
+    {
+        [Required(ErrorMessage = "Url is required!!!")]
+        [CustomValidation(typeof(AddSubmodule), nameof(ValidateURL))]
+        public string Url
+        {
+            get => _url;
+            set => SetProperty(ref _url, value, true);
+        }
+
+        [Required(ErrorMessage = "Reletive path is required!!!")]
+        [CustomValidation(typeof(AddSubmodule), nameof(ValidateRelativePath))]
+        public string RelativePath
+        {
+            get => _relativePath;
+            set => SetProperty(ref _relativePath, value, true);
+        }
+
+        public bool Recursive
+        {
+            get;
+            set;
+        }
+
+        public AddSubmodule(Repository repo)
+        {
+            _repo = repo;
+            View = new Views.AddSubmodule() { DataContext = this };
+        }
+
+        public static ValidationResult ValidateURL(string url, ValidationContext ctx)
+        {
+            if (!Models.Remote.IsValidURL(url))
+                return new ValidationResult("Invalid repository URL format");
+            return ValidationResult.Success;
+        }
+
+        public static ValidationResult ValidateRelativePath(string path, ValidationContext ctx)
+        {
+            if (Path.Exists(path))
+            {
+                return new ValidationResult("Give path is exists already!");
+            }
+
+            if (Path.IsPathRooted(path))
+            {
+                return new ValidationResult("Path must be relative to this repository!");
+            }
+
+            return ValidationResult.Success;
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = "Adding submodule...";
+
+            return Task.Run(() =>
+            {
+                var succ = new Commands.Submodule(_repo.FullPath).Add(_url, _relativePath, Recursive, SetProgressDescription);
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return succ;
+            });
+        }
+
+        private readonly Repository _repo = null;
+        private string _url = string.Empty;
+        private string _relativePath = string.Empty;
+    }
+}