Clause tree accessibility

[juleskuehn]

The step 2 clause tree needs one critical fix, and another should be considered:

1. Checkboxes currently do not appear as checkboxes to AT. (due to use of aria-hidden="true"). Resolving this issue while maintaining full keyboard support is more complicated than removing the aria-hidden attribute. Careful use of role and tabindex with the nested UI elements may be involved in the solution.
2. Keyboard commands for navigating the clause tree, while following ARIA defined patterns, are not always intuitive. Although instructions are provided within the "Instructions: Step 2" modal, some users may not open this modal. Consider providing an accessible popup tooltip etc. if the clause tree is focused by keyboard for the first time (use a cookie to prevent users from constantly seeing this).