move tabs to globalnav plugin

[scottjehl]

tabs can possibly just be a simple styled list of links that sit in the header/footer, as a sibling to all .ui-page containers. Clicking them will be like clicking any other local or ajax link, except that the global nav tabs will persist while pages animate beneath it.
This approach would let us cut a lot of code, as tabs are quite complex when dealing with aria roles, focus and keyboard management, etc