popover_menus: Extend arrow based navigation to tabbable components.

Instead of only checking for visible `a` tags, we should also check
for visible elements with the `tabindex=0` attribute defined. This
allows us to support navigation for other components which we
explicitly set to be tabbable.

web/src/popover_menus.ts

@@ -146,7 +146,7 @@ function get_popover_items_for_instance(instance: PopoverInstance): JQuery | und
         return undefined;
     }
 
-    return $current_elem.find("a:visible");
+    return $current_elem.find("a, [tabindex='0']").filter(":visible");
 }
 
 export const default_popover_props: Partial<PopoverProps> = {