a11y: Create accessible Menu component

[nwmac]

We have several menus in the UI and no common Menu component, e.g.:

• Locale selector on Login Page
• Page-level menu on Home Page
• User Avatar menu
• ..

We should create a Menu component that is keyboard accessible and shows focus state appropriately.

This can then be placed in to the above places to help address accessibility issues, rather than fixing them multiple times.

Check w3c guidelines for correct behaviour of a menu:

• https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-navigation/
• https://www.w3.org/WAI/tutorials/menus/

(Note, these seem to be contradictory, so need to make sure we have the correct pattern)

[aalves08]

From my work done on accessibility so far, there are some items to replace after this component is created:

• LocaleSelector menu a11y: Fix keyboard navigation, role, aria-label and focus display issues on the login screen #12775
• User menu header a11y: Fix keyboard navigation, aria label/role and focus display for user menu #12790
• Action menu header a11y: Fix keyboard navigation, aria-labels, roles and focus display for page-level action menu #12772
• Sortable table row actions menu -> which is the same ActionMenu component as the settings page a11y: Fix keyboard navigation and focus display issues on the home screen #12776 + a11y: Fix keyboard navigation and focus display for Global Settings #12787
• NamespaceFilter ? is not a v-dropdown, but it's a custom element... Probably we should replace it 🤔 a11y: Fix keyboard navigation, aria-label, role and focus display issues on the namespace chooser control #12779
• Bulkable Actions

Will keep editing this list as my work on accessibility progresses

[aalves08]

@rak-phillip found a new v-dropdown, tied to the header work (there's no specific issue, I think):

https://github.com/rancher/dashboard/blob/master/shell/components/nav/WindowManager/ContainerLogs.vue#L622-L661