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
yeah, that's ok.  I would have just removed the attribute but if we add classes somewhere else we would need to fix this.  jQuery should be the one to clean this up.