Mouse: Fix behavior for autocompletion UI

[bryphe]

• Enable clicking autocompletion items
• Add hover effect for autocompletion items
• Fix the cursor/pointer for autocompletion items, to suggest that they are clickable.

[bryphe]

This would also be a good opportunity to add a scrollbar (tackle #166), and leverage something like react-virtualized to only render a subset of the list.