refactor: rewrite Launcher

* move main tabbar to a standalone control
* simpfy notification

src/ViewModels/Notification.cs

@@ -0,0 +1,13 @@
+namespace SourceGit.ViewModels
+{
+    public class Notification
+    {
+        public bool IsError { get; set; } = false;
+        public string Message { get; set; } = string.Empty;
+
+        public void CopyMessage()
+        {
+            App.CopyText(Message);
+        }
+    }
+}