Added 2 new buttons for prev/next change in Diff

These new buttons in DiffView toolbar are visible when IsTextDiff.
They invoke new (and currently empty) methods PrevChange() / NextChange() in DiffContext.

Author: goran-w

src/ViewModels/DiffContext.cs

@@ -73,6 +73,16 @@ public DiffContext(string repo, Models.DiffOption option, DiffContext previous =
             LoadDiffContent();
         }
 
+        public void PrevChange()
+        {
+            // To be implemented...
+        }
+
+        public void NextChange()
+        {
+            // To be implemented...
+        }
+
         public void ToggleFullTextDiff()
         {
             Preference.Instance.UseFullTextDiff = !Preference.Instance.UseFullTextDiff;