EuiComboBox: Retain focus on input as you traverse the options with the keyboard

[cjcenizal]

Per #567 (comment):

I think we should separate the up/down selection from the input focus. Right now you can't move up and down and type at the same time. I think you want to keep the cursor focused in the input as they move up and down. I know that means you don't get the benefit of auto-scrolling because of the item focus, but having them as separate actions means your hands need to do much less work to move around. With the aria stuff I think you'd have to read the selection independently, but you can see the benefit there too, since it will read the first match as you type.

Tied to that, I'd auto-select the first option so you can just jam on enter immediately. As mentioned before, should also help speed up a screen reader's time to action as well.

We can implement this by applying a "focused" class to the option instead of using actual browser focus. We'd have to implement some logic to scroll the list to the new active option.

[chandlerprall]

Related to #1195 , the fix for which is to keep focus in the combobox's input.