Add UIA support for radio buttons and radio button groups

[mwcampbell]

These controls require us to implement the UIA selection and selection item patterns, as with list boxes in #23. Re-use as much of the code for implementing these patterns as possible, while still accounting for differences between these control types.

[mwcampbell]

#90 partially addresses this. I don't think I have to actually implement the UIA selection pattern to support radio groups, but we do need to compute position and count from the radio_group field on a given radio button. Leaving this open for now.