Since we don't use real NSMenus in the bookmark bar, the submenus do not behave correctly in VoiceOver.

Nonexhaustively:
- Elements don't have the correct roles (Menu, MenuItem etc.)
- VoiceOver keys don't behave correctly navigating to/from submenus and up/down menus
- VoiceOver focus doesn't follow keyboard focus

A first step would be to reverse engineer the hierarchy of regular non-menu-bar nested menus (for example Safari bookmark bar) and find a way to impose a similar hierarchy on the bookmark bar.