[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 
i would just do all of this updating in `pane.js` there is `activePaneChange` event that is already being listened to for the "active/inactive" state.  you can listen `paneCreated` and `paneDestroy` events and use `MainViewManger.getPaneCount()` to determine if you need to show the header.