[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
- [x] Italicize header text

@JeffryBooher 
Yes, but that's not enough. `paneCreate` event is too early for initial DOM update, so also have to do this in `_initialize()`. Also need to list for "resize" event of main window. Done.