Implement Pane Headers

This is a squashed version of PR #9051 (branch randy/splitview-pane-header)

cc @JeffryBooher 

You should just do:

``
_.forEach(_panes, function(pane) [
          pane.updateHeader();
});

```
And just do the work of `showHeader` and `updatePaneSIze` in `updateHeader`. In that one instance below where you're just calling `updatePaneSize` calling `updateHeader` would just resize since it would compute whether or not to show or hide based on `MainViewManger.getPaneCount()` and the current state of the header's visibility.

We should probably get a handle on what needs to be done wrt pane resizing.   Probably a call to `_updatePaneHeaders()` during `_updateLayout()` but I'm not sure.  Should we get that pr in first so you can test it?