Add support for horizontal scrolling in tab-line

* lisp/tab-line.el ([tab-line wheel-left]): Bind left and right tab
line scrolling to the 'wheel-left' and 'wheel-right' mouse events.
([tab-line S-wheel-right]): Analogous change for the functions that
switch tabs.
* etc/NEWS: Announce the new feature (bug#43224).
Copyright-paperwork-exempt: yes

etc/NEWS

@@ -206,6 +206,11 @@ of the next command to be displayed in a new tab.
 Show/hide the tab bar independently for each frame, according to the
 value of 'tab-bar-show'.
 
+---
+*** The tabs in the tab line can now be scrolled using horizontal scroll.
+If your mouse or trackpad supports it, you can now scroll tabs when
+the mouse pointer is in the tab line by scrolling left or right.
+
 ** New bindings in occur-mode, 'next-error-no-select' bound to 'n' and
 'previous-error-no-select' bound to 'p'.
 

lisp/tab-line.el

@@ -764,11 +764,15 @@ from the tab line."
 (global-set-key [tab-line mouse-5]    'tab-line-hscroll-right)
 (global-set-key [tab-line wheel-up]   'tab-line-hscroll-left)
 (global-set-key [tab-line wheel-down] 'tab-line-hscroll-right)
+(global-set-key [tab-line wheel-left] 'tab-line-hscroll-left)
+(global-set-key [tab-line wheel-right] 'tab-line-hscroll-right)
 
 (global-set-key [tab-line S-mouse-4]    'tab-line-switch-to-prev-tab)
 (global-set-key [tab-line S-mouse-5]    'tab-line-switch-to-next-tab)
 (global-set-key [tab-line S-wheel-up]   'tab-line-switch-to-prev-tab)
 (global-set-key [tab-line S-wheel-down] 'tab-line-switch-to-next-tab)
+(global-set-key [tab-line S-wheel-left] 'tab-line-switch-to-prev-tab)
+(global-set-key [tab-line S-wheel-right] 'tab-line-switch-to-next-tab)
 
 
 (provide 'tab-line)