ARIA attribute for value on combobox role

[smhigley]

Right now, there is no way to specify a programmatic value separate from the name for comboboxes. This is an issue when creating a combobox out of something other than an <input> element.

Aside from the theoretical need, this means creating a select-only combobox requires either using an <input readonly> (undesired, since it is not readonly), or combining the value and name together.

aria-valuetext seems like it might be a reasonable solution, if it were allowed on combobox.

Thoughts? (This relates to the 3/17 aria-practices tf discussion on a select-only combobox example)

[JAWS-test]

This applies not only to combobox, but also to other form fields with value, such as textbox, see #711

[jnurthen]

@smhigley Are you happy to close this and use #711 ? Please reopen if that doesn't work for you.