Make context menu accessible

[smalos]

Keyboard shortcuts should be added to provide easier navigation for people with disabilities.

• Esc: Closes the opened ContextMenu.
• Enter: Selects the focused item.
• Up: Navigates up or to the previous menu item.
• Down: Navigates down or to the next menu item.
• Left: Close the current sub menu and navigates to the parent menu.
• Right: Navigates and open the next sub menu.

[nkappler]

Hi @smalos,
sorry for the long delay without updates.
I am currently very busy due to some new projects at my workplace...
At the moment I don't know when I'll be able to start working on this, so if you need this feature or if you want to help, I am happy to review any Pull Requests and appreciate any help :)

[smalos]

@nkappler

If you look at jQuery contextMenu, you will see that keyboard navigation works there, even though the menu items do not appear to have a tabindex.

You might get some inspiration from how they did it.