MDL-70792 output: Menu items should have -1 tab index

* As per WAI ARIA Authoring Practices 1.1 guidelines for menus,
menu items should have a tab index of -1. Navigation between menu items
is done via arrow keys.

See https://www.w3.org/TR/wai-aria-practices-1.1/#menu

lib/outputcomponents.php

@@ -4321,6 +4321,7 @@ public function add($action) {
     public function add_primary_action($action) {
         if ($action instanceof action_link || $action instanceof pix_icon) {
             $action->attributes['role'] = 'menuitem';
+            $action->attributes['tabindex'] = '-1';
             if ($action instanceof action_menu_link) {
                 $action->actionmenu = $this;
             }
@@ -4336,6 +4337,7 @@ public function add_primary_action($action) {
     public function add_secondary_action($action) {
         if ($action instanceof action_link || $action instanceof pix_icon) {
             $action->attributes['role'] = 'menuitem';
+            $action->attributes['tabindex'] = '-1';
             if ($action instanceof action_menu_link) {
                 $action->actionmenu = $this;
             }
@@ -4393,7 +4395,8 @@ public function get_primary_actions(core_renderer $output = null) {
             'title' => $title,
             'aria-label' => $label,
             'id' => 'action-menu-toggle-'.$this->instance,
-            'role' => 'menuitem'
+            'role' => 'menuitem',
+            'tabindex' => '-1',
         );
         $link = html_writer::link('#', $string . $this->menutrigger . $pixicon, $attributes);
         if ($this->prioritise) {