Implement Pane Headers

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

can't we resize the panes in  `_updateLayout()`  We're going to need to do that for resize panes plus if there is a workspace panel open that changes the view that doesn't change the size of the window you will need to capture that like when the `sidebar` changes size.  That's already handled in `_updateLayout()`