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/CreateGroup.cs

@@ -0,0 +1,38 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class CreateGroup : Popup
+    {
+        [Required(ErrorMessage = "Group name is required!")]
+        public string Name
+        {
+            get => _name;
+            set => SetProperty(ref _name, value, true);
+        }
+
+        public CreateGroup(RepositoryNode parent)
+        {
+            _parent = parent;
+            View = new Views.CreateGroup() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            Preference.AddNode(new RepositoryNode()
+            {
+                Id = Guid.NewGuid().ToString(),
+                Name = _name,
+                IsRepository = false,
+                IsExpanded = false,
+            }, _parent);
+
+            return null;
+        }
+
+        private readonly RepositoryNode _parent = null;
+        private string _name = string.Empty;
+    }
+}