Clean up pagebreak code, fixing notices and nested pagebreaks

The patch at #1507718: Return entire form if $filter = FALSE in webform_client_form() to simplify Form Builder integration allows us to simplify our pagebreak code. Right now we fake pagebreaks by creating "pagebreak_clone" components that don't really exist, which results in throwing a PHP Notice in Webform module because the component type doesn't really exist. It also results in funky handling in form_builder_webform.module.

This patch builds on #1507718: Return entire form if $filter = FALSE in webform_client_form() to simplify Form Builder integration to simplify our page handling. It also prevents errors from occurring when users try to nest a pagebreak inside of fieldsets, though that fix ideally would be corrected at the JavaScript layer to prevent users from nesting in the first place.