Part of bgo#613728 - [GtkTreeView] Focus the list, not the headers, when Tab-focusing

Part of bgo#613728 - [GtkTreeView] Focus the list, not the headers, when Tab-focusing

The idea is that it is way more common to want to manipulate the
actual list, rather than the headers.  Once you Tab into the treeview
(and the list part gets the focus), you can use Shift-Tab to focus
the headers.

This means that some hysteresis is added to the focus chain, but
it makes the treeview a lot more convenient to focus with the keyboard.

Signed-off-by: Federico Mena Quintero <federico@novell.com>