* lisp/tab-line.el: More configurability for tab buffer groups.

* lisp/tab-line.el: More configurability for tab buffer groups.

* lisp/tab-line.el (tab-line-tabs-buffer-group-function)
(tab-line-tabs-buffer-group-sort-function)
(tab-line-tabs-buffer-groups-sort-function): New defvars.
(tab-line-tabs-buffer-group-name): Rename from
tab-line-tabs-buffer-groups--name and use tab-line-tabs-buffer-group-function.
(tab-line-tabs-buffer-groups): Use tab-line-tabs-buffer-groups-sort-function
and tab-line-tabs-buffer-group-sort-function.
(tab-line-new-tab): Let bind tab-line-tabs-buffer-groups to
mouse-buffer-menu-mode-groups.