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/Checkout.cs

@@ -0,0 +1,35 @@
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class Checkout : Popup
+    {
+        public string Branch
+        {
+            get;
+            private set;
+        }
+
+        public Checkout(Repository repo, string branch)
+        {
+            _repo = repo;
+            Branch = branch;
+            View = new Views.Checkout() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = $"Checkout '{Branch}' ...";
+
+            return Task.Run(() =>
+            {
+                var succ = new Commands.Checkout(_repo.FullPath).Branch(Branch, SetProgressDescription);
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return succ;
+            });
+        }
+
+        private readonly Repository _repo;
+    }
+}