[Review Only] Randy/splitview pane header

Replace pane border highlighting with header.

TODO:
- [ ] Fix editor sizing so horizontal scroll bars are not truncated from bottom
- [ ] Fix empty editor to show logo, italicize header text

@JeffryBooher 
we should cache `pane.$el.find(".pane-editor-holder")` also the name shouldn't be `editor-holder` since it can host images and other things.  maybe `pane-content`