Adding header and footer items for combobox.

[davidlygagnon]

Fixes #1600

Picking up left over work from #1638 where we want to have the new optionsSearchEntity and optionsAddItem working with keyboard navigation (highlighted and actionable).

Additional description

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[interactivellama]

I think the search entity is supposed to be highlighted on typing. I think that is why it's a new prop. It's allow supposed to update with the input value in the menu item in the example, too.

[interactivellama]

@davidlygagnon Is this ready for review?

[davidlygagnon]

@interactivellama Yes - this is ready for review. Thanks.