[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 
Don't need to do anything in `pane._notifyCurrentViewChange()` because the highlights ar all handled in CSS. I added `pane._updatePaneHeaderText` to update header text when file selection changes.