Per discussion on #2089, implement the following:

• A Timber\PagesMenu class.
• A timber/pages_menu/classmap filter.
• A Timber::get_pages_menu() function.
• A Timber\PagesMenuFactory class - used under the hood in ::get_pages_menu(), calls the timber/pages_menu/classmap filter.
• Make Menu and MenuItem constructors protected.
• Draft the relevant documentation as well as the notes in the Upgrade Guide for this.

There is already a test-timber-pages-menu.php test file. We may want to flesh this out with a few more tests, or redesign some of the existing tests.