Multiselect: refactor drop-down transition to issue proper event states

[anselmbradford]

Describe the bug
The search input opens when the search input gets focus and closes when it's blurred. Focus happens on mousedown, so it gets the attributes of being open when it is clicked, not when the drop-down has actually completed opening. We should re-write this to dispatch events for the life cycle of the drop-down expand (expandBegin, transitionBegin, transitionEnd, expandEnd) and (collapseBegin, transitionBegin, transitionEnd, collapseEnd). The attributes of the drop-down being open should be added at the end of the expandEnd sequence and removed at the beginning of the collapseBegin sequence. Currently, the collapse method (erroneously) dispatches the expandEnd event.

[sonnakim]

We talked about making the multiselect a web component, this could be addressed as part of that work