bug#69578: 30.0.50; tab-bar-mode binding of (control tab) not always useful

[Juri Linkov]

> There is a slight backward incompatibility for users who have own
> global keybindings for C-TAB.  They will need to unbind these keys
> in the new map.  Then this change should be announced in NEWS:
>
> diff --git a/etc/NEWS b/etc/NEWS
> +---
> +*** New keymap 'tab-bar-mode-map'.
> +If you have global keybinding for 'C-TAB', then you might want
> +to unbind the same keybinding in 'tab-bar-mode-map'.

I still feel uneasy about this backward incompatibility
since users will suddenly get tab-switching on C-TAB
when they use another global keybinding for C-TAB.
So we need to rethink this change.