MDL-40975 ActionMenu: Ensure we maintain consistent tab order

When the actionmenu is open, we should focus on it's first element, and
then naturally tab through it. Tabbing from the end should take us to the
first element after the menu button. This may not be the same as the first
element after the menu itself because of the nature of primary and
secondary action links. We also need to shift-tab back in the same manner.