Fix ComboBox (dropdown) bugs

[aswallace]

Addresses the following bugs specific to the ComboBox (FluentUI's version of a dropdown with search)

• When the dropdown options were open, using arrow keys to navigate the options also changed the file selection in the background (very bad!!)
• Clicking inside & typing didn't open the menu by default. It now opens when you start typing (but not on click alone unfortunately)
• Added styling in combobox menu for focus/active/selected
• Searched item was not always appearing at the top of the search results. It now does unless it's the last item in the list

Also had to remove fuzzy search/fuse from the dropdown menu since it wasn't playing well with the built-in autocomplete, was not a requested feature, and was taking too long to make work.

closes #342

Styling for active/focused menu items (e.g., while hovering or using arrow keys to navigate)