Add ComboBox Widget

[ecton]

A useful widget is a combination of a text input and a dropdown list. Usually the dropdown list in a combobox is a "ListBox" type widget, which introduces this issue dependency:

• Blocked by: Add Selection + Interactivity to Grid #100

We may need a little more control over sizing the dropdown overlay to make the list look as good as it can, assuming all of the contents fit.

This widget's text input mode should support two modes of operation:

• Strict entry where only items that are in the list are allowed. Typing in the input acts as a type-ahead filter similar to what we need to implement in Add keyboard accessibility to Menu #150
• Fuzzy entry where items autocomplete but arbitrary entry is allowed