feature: enhance the behaviour of closing the last tab

* If the last tab is an opened repository, go back to the welcome page
* If the last tab is welcome page, quit this app

src/ViewModels/LauncherPage.cs

@@ -26,13 +26,7 @@ namespace SourceGit.ViewModels
 
         public LauncherPage()
         {
-            _node = new RepositoryNode()
-            {
-                Id = Guid.NewGuid().ToString(),
-                Name = "WelcomePage",
-                Bookmark = 0,
-                IsRepository = false,
-            };
+            _node = new RepositoryNode() { Id = Guid.NewGuid().ToString() };
             _data = new Welcome();
         }