This controls which key bindings tab-bar creates. Values are t,the
default, which defines all keys and is backwards compatible, 'numeric
(tab number selection only), 'tab (TAB and SHIFT-TAB keys only), nil
(which defines none).

This is useful to avoid key binding conflicts, such as when folding in
outline mode using TAB keys, or when a user wants to define her own
tab-bar keys without first having to remove the defaults.

-Stephane