consider adding WAI-ARIA attributes to the header dropdown

[Aleksej]

I don't understand what I'm doing, but here is a patch that adds aria-controls attribute that, according to a candidate recommendation (​http://www.w3.org/TR/wai-aria/states_and_properties#aria-controls), is supposed to tell some software that the down/up arrow icon controls the header drop-down.

At least it *hopefully* doesn't break anything and doesn't add any requirements for more attributes.

[Aleksej]

patch (adds aria-control and changes a class attribute to id accordingly)

[rodney757]

What is the reason for changing the class to an id?

[Aleksej]

​http://www.w3.org/TR/wai-aria/states_and_properties#aria-controls says "Value: ID reference list", linking to ​http://www.w3.org/TR/wai-aria/states_and_properties#valuetype_idref_list which says "ID reference list: A list of one or more ID references." and "ID reference: Reference to the ID of another element in the same document".

[Christopher Allan Webber]

Good patch. All that was missing was adjusting the CSS properties.

I committed this in commit 4b24678... thank you!