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
When switching back to "No Split", the editor holder tag has an empty class attribute:

```html
    <div id="editor-holder" style="height: 957px;" class>
```