GtkNotebook: Fix reverse tab order

GtkNotebook: Fix reverse tab order

When using Shift-Tab to move the focus out of page content onto
the tab label, we end up in a situation where both Tab and Shift-Tab
move focus back into the page, which is not really what is expected
when the notebook is part of a dialog.

Instead, arrange things so that using Shift-Tab with focus on a
tab label moves the focus out of the notebook.

http://bugzilla.gnome.org/show_bug.cgi?id=669986