Move closure filling to the actual end

Themes output the hook_footer() output on the bottom of the page, and this is the latest time we can do something to output on the page. Unfortunately theme.inc fills the closure variable (with output from hook_footer()) before doing lots of parts of the page. The theme variables filled were ordered in alphabetical order, which put the closure filling to a bad place.

For hook_footer() to be able to provide a list of db queries used on the page (devel module), or provide a list of locale strings used on the page (l10n_client module), the closure should be filled in last. The attached patch does just this, and because I moved the closure out of the alphabetical order, I added a small comment.