(A11y severity 4) Improve accessibility of tabs

[sathomas]

Accessibility of the tabs in the "Library" section, as well as the "Add to folder" modal, could be enhanced by using an ARIA tab panel (role="tab", see http://hanshillen.github.io/jqtest/#goto_tabs for an example). If implemented, a screen reader user would be notified that a group of tabs is present and keyboard users could navigate through the tabs using the arrow keys instead of the tab keys.