Add block prop to GlListbox

Problem:

With the legacy GlDropdown there was an easy way to display a dropdown block. Not much thinking involved, add the prop and move on. We are missing that functionality now with GlListbox. Users would need to add custom CSS to the component that's not very straightforward since several elements would need to be targeted (actually not even sure if it's possible at the moment).

Applying classes that used to work on GlDropdown no longer work on GlListbox resulting in

Solution:

Extend GlListbox component API to support block or something similar.