Editor Utility Widgets are reset every time another blueprint is reinstanced

Editor utility widgets were being rebuilt on GEditor->OnBlueprintReinstanced. However, this delegate is fired when any blueprint is reinstanced, which resulted in the widget being refreshed when it hadn't been changed.

1. Create an editor utility widget. Add a checkbox
2. Run the widget and check the box
3. Open another blueprint and recompile it. Note that the EUW checkbox is now unchecked again