[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 it's in pane then `MainViewManager` doesn't needs to know the DOM structure of `Pane` objects or how the header is implemented. Hiding and showing the header should be private to the pane -- we shouldn't encourage external DOM manipulation or it becomes more difficult to maintain.

Each should be responsible for updating its own header.