Add UIA support for tabs

[mwcampbell]

A tab bar is like a list box in that an item (in this case a tab) is selected. But unlike list boxes, it's common to change the selected tab without actually moving focus to the tab bar, e.g. using a keystroke like Ctrl+Tab. So we'll need to make sure that all Windows screen readers announce this type of change. That should be a simple matter of raising the appropriate UIA selection event, but it's worth doing some extra testing here.