Feature: #89496: Make context menu usable via keyboard

See Issue #89496

Description

The context menus are now usable via keyboard. Pressing Shift+F10 will open the context menu for the focused element. It is also possible to use arrows, home and end keys in order to navigate through the menu. Besides that, using enter and space keys will active items or open submenus.

This change follows the best practices as described in WAI-ARIA Authoring Practices 1.1, see the W3 document for further reading.

Impact

Added tabindex, role, and aria-* attributes to context menus as advised in WAI-ARIA Authoring Practices 1.1. Screen readers are now able to recognize the context menu properly.