[RFC] Submenu functionality in Menu component

[jurokapsiar]

Feature Request

Problem description

Menu should allow defining submenus. Each menu item can have a submenu.
Submenu should follow ARIA specification and best practices:
https://www.w3.org/TR/wai-aria-practices/#menu
https://www.w3.org/TR/wai-aria-practices/examples/menubar/menubar-1/menubar-1.html

especially with regards to the rendered DOM strucutre and keyboard navigation:

• Submenu should be rendered as <ul> inside of the MenuItem wrapper slot. Menu items of that submenu should render normally as <li> items.
• Submenu is always vertical
• Home and End keys should not be used as the menu implements circular navigation.

As opposed to the specification described in the links above, submenu should not open when the user hovers on the menu item with a submenu. It should open when the user clicks on such a menu item or if he uses keyboard navigation keys for subemnu open.

API Proposal

• add menu prop to the MenuItem component. User can pass a Menu shorthand to it.

MVP

There has already been two PRs implementing this. Both had good and bad sides and the good parts were merged into #539.