refactor: add a popup panel to show submodule updating status

src/ViewModels/UpdateSubmodules.cs

@@ -0,0 +1,28 @@
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class UpdateSubmodules : Popup
+    {
+        public UpdateSubmodules(Repository repo)
+        {
+            _repo = repo;
+            View = new Views.UpdateSubmodules() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = "Updating submodules ...";
+
+            return Task.Run(() =>
+            {
+                new Commands.Submodule(_repo.FullPath).Update(SetProgressDescription);
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return true;
+            });
+        }
+
+        private readonly Repository _repo = null;
+    }
+}