New keymap tab-line-mode-map and new tab order on tab-line (bug#69993)

New keymap tab-line-mode-map and new tab order on tab-line (bug#69993)

* lisp/tab-line.el (tab-line-new-button-functions): New variable.
(tab-line-tabs-function): Change the default value from
'tab-line-tabs-window-buffers' to the new option
'tab-line-tabs-fixed-window-buffers'.
(tab-line-tabs-buffer-group-sort-function): Change the default
value from nil to 'tab-line-tabs-buffer-group-sort-by-name'.
(tab-line-tabs-buffer-group-sort-by-name): New function.
(tab-line-tabs-fixed-window-buffers): New function.
(tab-line-format-template): Use 'tab-line-new-button-functions'.
(tab-line-mode-map, tab-line-switch-repeat-map): New keymaps.