* lisp/tab-line.el: New option for tabs where buffers are grouped by mode.

* lisp/tab-line.el: New option for tabs where buffers are grouped by mode.

* lisp/tab-line.el (tab-line-tabs-function): Add option
tab-line-tabs-buffer-groups.
(tab-line-tabs-buffer-groups): New defvar defaulted to
mouse-buffer-menu-mode-groups.
(tab-line-tabs-buffer-groups--name, tab-line-tabs-buffer-groups):
New functions.
(tab-line-format): Support tabs in the format '(tab (name . "name") ...)'.
(tab-line-select-tab): Move part of code to tab-line-select-tab-buffer.
(tab-line-select-tab-buffer): New function.
(tab-line-tab-current): Rename from tab-line-tab-selected.