add Option Categories

[justjake]

Add option categories, so we can present users with different sorts of options. This is well-supported in other autocomplete widgets.

This will require abstracting away the code to get widget DOM elements a bit. Right now you can add arbitrary elements to the combobox, but focusNext() and focusPrevious() will focus them, and they can then be selected.

[CMTegner]

Support for this landed a while back. You now have separate control over the menu and each menu item, allowing Autocomplete to only listen to the events (hover, click, etc) it wants to act upon.