Install pages enhancements to help reduce conflicts with other eCommerce plugins.

When attempting to install a new page:

* only search for pages (ignore posts and other custom post types)
* search for an existing page with the specified page content (typically a shortcode), or if no page content is specified, search for an existing page with the specified page slug.

Should fix #3100 and will also help ensure that WooCommerce only attempts to reuse existing pages that have the WooCommerce shortcodes on them, thus reducing conflicts with other eCommerce plugins.