Make scope bar buttons AXCheckbox'es for accessibility

This notifies the user the choice is exclusive to only one button. Also
including these buttons in an AXRadioGroup adds positional indication to the
radio buttons (like "2 of 3") which gives sense of how many values there are
to choose from, where in the list of those the user currently is, and, if
there are more sequential AXRadioGroups (which is not the case here), it
distinguishes in which one the user is making a choice.