UI: Add missing locale text for reorder undo/redo

UI/data/locale/en-US.ini

@@ -294,6 +294,7 @@ Undo.ShowTransition="Show Transition on '%1'"
 Undo.HideTransition="Hide Transition on '%1'"
 Undo.ShowSceneItem="Show '%1' in '%2'"
 Undo.HideSceneItem="Hide '%1' in '%2'"
+Undo.ReorderSources="Reorder sources in '%1'"
 
 # transition name dialog
 TransitionNameDlg.Text="Please enter the name of the transition"