Fix aria role for GlTabs

Ported over from gitlab-org/gitlab-ui!5008 (closed):

Fix aria role for GlTabs

Fixing aria roles for GlTabs and moving actions to a new #toolbar slot to improve the accessibility of the ul list

Screenshots or screen recordings

Before	After

Test: http://localhost:9001/iframe.html?args=&id=base-tabs--default&viewMode=story

Docs: https://www.w3.org/TR/2017/NOTE-wai-aria-practices-1.1-20171214/examples/tabs/tabs-2/tabs.html

Integrations

Integration branch: gitlab-org/gitlab!193415 (merged)