ux: add a warning icon when the tracking upstream of a local branch is gone (#1006)

Co-authored-by: Davide Tentori <dtentori@softeam.it>

src/Models/Branch.cs

@@ -34,6 +34,7 @@ namespace SourceGit.Models
         public string Upstream { get; set; }
         public BranchTrackStatus TrackStatus { get; set; }
         public string Remote { get; set; }
+        public bool IsUpsteamGone { get; set; }
 
         public string FriendlyName => IsLocal ? Name : $"{Remote}/{Name}";
     }