Settings Button Keyboard Navigation Improvements

Problem/Motivation

When navigating the settings buttons via keyboard, there are some things that can be improved:
* The settings dropdown can be opened and closed by hitting enter when the button has focused, and it is possible to then tab into the dropdown list. However it is not possible to navigate using the arrow keys.
* When tabbing through the dropdown menu it highlights the entire list item (or the link possibly) and then the individual checkbox. I'm not positive, but that seems off. We could compare to similar checklists elsewhere in Drupal. When tabbing through our content type settings for example only the checkboxes are highlighted.
* Active highlighting for the new window button is different from the settings and full screen buttons. New window wraps tightly around the icon and the text while the others highlight the entire button.

And one thing I'm not sure about - when tabbing past the view mode dropdown it cycles back up to the close button and then the settings button. Should it be possible to tab into the actual preview iframe from there?