Incompatibility with modules that print theme('page') directly

Certain modules (like panels) print theme('page') directly to the output rather than returning it in the page callback function. This screws up popups pretty bad since it depends on menu_execute_active_handler() returning something useful. Also this output is essentially prepended to the json that is returned to the browser from popups_render_as_json().

This patch does a couple of things to clean up these related issues.

1. The call to menu_execute_active_handler() is buffered so that if modules print from the page callback, it does not get sent to the browser.
2. In popups_preprocess_page(), if the page_override session variable is set, then we populate a special region, 'popups_content', with the $content variable passed to theme('page'). This gives us a clean content section, regardless of how theme('page') is invoked.
3. When $content is null, $content then gets the contents of the special 'popups_content' region that was populated in popups_preprocess_page().