Accessibility: Elastic UI Context Menu

[aphelionz]

Subset of tickets in #614 that deal with the "Context Menu" category of EUI, anything that falls on or under this URL: https://elastic.github.io/eui/#/navigation/context-menu

• Context Menu Items should be nested within an ARIA-compatible menu stru cture [EuiContextMenu] Items should be nested within an ARIA-compatible menu structure #731
• Context menu needs instructions on how to exit, see Color Theme Picker Keyboard Trap Color Theme Picker Keyboard Trap #619

[myasonik]

Closing this meta as it only tracks one issue