* lisp/tab-line.el (tab-line-tabs-buffer-list-function): New variable.

* lisp/tab-line.el (tab-line-tabs-buffer-list-function): New variable.

(tab-line-tabs-buffer-list): New function.
(tab-line-tabs-mode-buffers, tab-line-tabs-buffer-groups):
Call tab-line-tabs-mode-buffers.
(tab-line-tabs-buffer-groups): Add 'close' function that uses
kill-buffer instead of bury-buffer that makes no sense here.
(tab-line-format): Don't show the close button when a tab
has no 'close' function or buffer.
(tab-line-switch-to-prev-tab, tab-line-switch-to-next-tab):
Support tabs with 'buffer' properties.
(tab-line-close-tab): Call 'close' function when defined.