Tab-line horizontal scrolling with UI buttons and commands (bug#37667)

Tab-line horizontal scrolling with UI buttons and commands (bug#37667)

* etc/images/tabs/left-arrow.xpm:
* etc/images/tabs/right-arrow.xpm: New images.

* lisp/tab-line.el (tab-line-left-map, tab-line-right-map): New keymaps.
(tab-line-left-button, tab-line-right-button): New variables.
(tab-line-tab-name-function): Turn defvar into defcustom.
(tab-line-tab-name-buffer): New function.
(tab-line-tab-name-truncated-buffer): Rename from tab-line-tab-name.
(tab-line-tabs-limit): Default to nil.
(tab-line-tabs): Behavior depends on tab-line-tabs-limit.
(tab-line-format): Use window-parameter tab-line-hscroll.
Add left/right buttons.
(tab-line-hscroll): New function.
(tab-line-hscroll-right, tab-line-hscroll-left): New commands
bound to mouse-wheel.  Rebind tab-switching commands to	mouse-wheel
with Ctrl-modifier.