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/CreateTag.cs

@@ -0,0 +1,78 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+using System.Text.RegularExpressions;
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class CreateTag : Popup
+    {
+        [Required(ErrorMessage = "Tag name is required!")]
+        [RegularExpression(@"^[\w\-\.]+$", ErrorMessage = "Bad tag name format!")]
+        [CustomValidation(typeof(CreateTag), nameof(ValidateTagName))]
+        public string TagName
+        {
+            get => _tagName;
+            set => SetProperty(ref _tagName, value, true);
+        }
+
+        public string Message
+        {
+            get;
+            set;
+        }
+
+        public object BasedOn
+        {
+            get;
+            private set;
+        }
+
+        public CreateTag(Repository repo, Models.Branch branch)
+        {
+            _repo = repo;
+            _basedOn = branch.Head;
+
+            BasedOn = branch;
+            View = new Views.CreateTag() { DataContext = this };
+        }
+
+        public CreateTag(Repository repo, Models.Commit commit)
+        {
+            _repo = repo;
+            _basedOn = commit.SHA;
+
+            BasedOn = commit;
+            View = new Views.CreateTag() { DataContext = this };
+        }
+
+        public static ValidationResult ValidateTagName(string name, ValidationContext ctx)
+        {
+            var creator = ctx.ObjectInstance as CreateTag;
+            if (creator != null)
+            {
+                var found = creator._repo.Tags.Find(x => x.Name == name);
+                if (found != null)
+                    return new ValidationResult("A tag with same name already exists!");
+            }
+            return ValidationResult.Success;
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = "Create tag...";
+
+            return Task.Run(() =>
+            {
+                Commands.Tag.Add(_repo.FullPath, TagName, _basedOn, Message);
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return true;
+            });
+        }
+
+        private readonly Repository _repo = null;
+        private string _tagName = string.Empty;
+        private readonly string _basedOn = string.Empty;
+    }
+}