Customizable tab-line-tabs-function and new buffer-local tab-line-mode.

Customizable tab-line-tabs-function and new buffer-local tab-line-mode.

* lisp/tab-line.el (tab-line-tabs-function): Turn defvar into defcustom
with default of tab-line-tabs-window-buffers.
(tab-line-tabs-mode-buffers): New function.
(tab-line-tabs-window-buffers): Rename from tab-line-tabs.
(tab-line-select-tab, tab-line-switch-to-prev-tab)
(tab-line-switch-to-next-tab): Handle the value tab-line-tabs-window-buffers
of tab-line-tabs-function specially.
(tab-line-mode): Rename from global-tab-line-mode and change scope
to buffer-local.
(global-tab-line-mode): New globalized-minor-mode.
(tab-line-mode--turn-on): New function.
(tab-line-exclude-modes): New defcustom.
(tab-line-exclude): New buffer-local variable.
https://lists.gnu.org/archive/html/emacs-devel/2019-10/msg00988.html

* lisp/dired.el (dired-mark-pop-up): Set tab-line-exclude to nil.
Don't set tab-line-format.   (Bug#37699)
* lisp/speedbar.el (speedbar-mode): Set tab-line-exclude to nil.
Don't set tab-line-format.