Add content type plugin to show a book node's children pages menu only

I've created a content type plugin for Ctools that allows the user to bring in a book node's children pages menu. It's based on the existing node_book_nav plugin.

Why an extra content type plugin ? The current book navigation plugin, by default, includes not only the children pages menu, but also a navigation "toolbar", all in one template file. Sure, we can make that template show only the children pages, but by doing so we lose the ability to easily bring in the "toolbar" somewhere else in the page. With this plugin, it makes it much easier to create a book panel page where the children pages menu and the navigation "toolbar" aren't next to each other (for example, you could have the children pages menu in the top, the node body in the middle, and the navigation toolbar at the bottom).