Calendar rebase (3) 

This replaces #1352.
Since this uses an existing id, we need to escape it when used in a selector (O Hai @arschmitz). See https://github.com/jquery/jquery-ui/blob/b6bec797d6a8ef0b377a866c38c67e66a626b45f/ui/tabs.js#L302.

We could store a reference to the element when we find it in `_setActiveDescendant()` so that we only need to escape it once.