[A11Y][UX] Admin bar: Allow navigation by keyboard arrows (besides Tab/Shift+Tab)

This has been annoying me for quite a while, but never actually filed an issue for it. When one makes a search via the Admin Bar, the results can only be navigated via the Tab/Shift+Tab keys. Up/down/left/right does not work.

Interesting read: Accessible Dropdown Menus and its successor Accessible Dropdown Menus Revisited (emphasis mine):

The WAI-ARIA 1.0 Authoring Practices site includes a standard set of Design Patterns, adopted from the DHTML Style Guide. These design patterns were developed by a group of key stakeholders to define a set of recommended models for how web-based menus and other interactive widgets should behave. In a nutshell, users should be able to tab into the menu, but once the menu has focus, they should be able to navigate left, right, up, and down using the arrow keys. Pressing enter, space, or down arrow on a top-level menu item should trigger the display of the dropdown or flyout menu if one is present, and pressing escape should close it and return focus to the parent menu item. When users tab out of the menu, any open sub-menus should close.

...jumping to a menu item by typing the letter that item starts with. This is a very cool feature, and saves users a lot of keystrokes especially if the menu is large.