Bug 877579 - Items in the awesome list flicker as the user types a URL

When the user types a URL in the browser, the awesome list suggests similarly spelled URLs/items. But the list is recreated for every keystroke, causing visual glitches (flickering and unnecessary processing and DOM reflows. Same happens for bookmarks and history, and even when users just click on tabs there, everything gets recreated and flickers.

This patch caches recently used items in the awesome bar to avoid reconstructing it every time with the same markup, in all cases.