Fix textbox selection when deleting

See https://forum.flaxengine.com/t/renaming-items-in-the-content-area-problem/334

Source/Engine/UI/GUI/Common/TextBoxBase.cs

@@ -620,8 +620,10 @@ namespace FlaxEngine.GUI
             {
                 var left = SelectionLeft >= 0 ? SelectionLeft : 0;
                 if (HasSelection)
+                {
                     _text = _text.Remove(left, selectionLength);
-
+                    SetSelection(left);
+                }
                 _text = _text.Insert(left, str);
 
                 SetSelection(left + str.Length);