Tooltip support for dropdown items in Context Menu and Menu Bar

[rolfsmeds]

Docs links for tooltip support:

• Context Menu: https://vaadin.com/docs/next/components/context-menu#tooltips
• Menu Bar: https://vaadin.com/docs/next/components/menu-bar#tooltips

The main use case for tooltips on these is for explaining why an item is disabled. Disabled items can be made focusable and hoverable (so that they can display tooltips) through a feature flag: accessibleDisabledMenuItems