[Review Only] Randy/splitview pane header

Replace pane border highlighting with header.

TODO:
- [x] Fix editor sizing so horizontal scroll bars are not truncated from bottom
- [x] Fix empty editor to show logo
- [ ] Italicize header text

@JeffryBooher 
can this be moved to `pane.js` and have it listen to `paneCreated` and `paneDestroyed` events?  Then it can use `MainViewManger.getPaneCount()` to determine if there is 1 or 2 panes to show/hide the header.