TableViewHeader: remove from tab chain
The TableViewHeader currently is part of the tab chain. Once it has keyboard focus, however, the user can do absolutely nothing there. To the user pressing tab to navigate the application by keyboard, it seems like the tab focus ends up nowhere. Having the header accept focus would make sense if the user could actually control the sort order of the table this way, but this would require larger changes to the way tables are handled.
This change removes TableViewHeaders from the tab chain. They can be included again once the work has been done to make them actually useful for keyboard users.