Add UIA support for drop-down lists (simple combo boxes)

[mwcampbell]

The top-level combo box control will need to implement the UIA value pattern to indicate the currently selected item. The drop-down list itself is like the list boxes described in #23. My intuition is that we'll need to pay special attention to making these work correctly with all of the Windows screen readers. But that's based only on my experience as a screen reader developer having issues with different combo box implementations, so maybe it's not as much of a problem in the other direction.

[mwcampbell]

I forgot: the top-level combo box control will also need to implement the expand/collapse pattern, to expose whether the drop-down list is opened or closed.