Add option to show selected items first in multi-select listboxes

Currently, when we select multiple items, they stay selected in whatever order they are on the list. We should add an option that allows us to group them into a "Selected" group at the top of the dropdown. This will match the designs, and a lot of the use-cases we have across the product. Right now, people are implementing their own ways to do this so we should standardize it.

We should make it an additional option though, as we don't want to break other folks current implementations.