This also fixes two issues I found whilst writing the tests:
1. Exclude hidden items from the set of navigable buttons and
2. Exclude disabled items from the set of navigable buttons whilst navigating,
because they may get disabled in the meantime (like with the edit controls).
MozReview-Commit-ID: 5WThVoTZjbV
--HG--
extra : rebase_source : e0bd85192e7b1043ecf04b7d9d3b546aadb004c9