a11y instrumentation of VerticalCheckboxGroup

[pixelzoom]

... which should be similar to VerticalAquaRadioButtonGroup. See #344 (comment).

[marlitas]

Meeting 11/5

• This is part of "Groups" need a consistent API for voicing/description #775
• Assigning @jessegreenberg for next steps.
• We do not want there to be a default for the accessible name for the group, however an option to add a group accessible name should be part of the API.