Add class to #editor-holder for split orientation

For #9146

@larz0 you can play with this branch

classes are:
"split-vertical"
"split-horizontal"

No class when there is only one pane

@redmunds can merge it if he likes it
You also want to make sure there's an orientation here:

```js
    if (orientation) {
        _$el.addClass("split-" + orientation.toLowerCase());
    }
```