refactor: move non-observable object from ViewModels to Models

* ViewModels.MergeMode -> Models.MergeMode
* ViewModels.Notification -> Models.Notification
* ViewModels.ResetMode -> Models.ResetMode
* use `int` instead of `ViewModels.CountSelectedCommits`

src/Models/Notification.cs

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