[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 
If we do it here then it happens for every `updateLayout` call.  If we do it in `paneCreated` / `paneDestroyed` then it will just happen once.