Override default page_template and post_parent

[aldenta]

In previous versions (not sure where this stopped working but I believe it worked in 2.6) you could override the default page_template and post_parent when creating a new post/page. This was extremely useful when using WP as a CMS. I had custom meta_boxes, would set the page_template to a specific template, set the post_parent to the correct parent, and then hide them with CSS so the user could not change them (hiding doesn't work now either since page_template and page_parent aren't wrapped in their own divs).

Now, based on how get_default_post_to_edit gets called, it sets the defaults and they cannot be changed. The attached diff adds two apply_filters that will allow hooking into this.

Thanks

[aldenta]

wp-admin/includes/post.php diff file

[Denis-de-Bernardy]

Closing as dup of #8223, which suggests a more general fix (without a patch)