Allow plugins to modify class value in page lists

[wnorris]

This patch adds a new filter 'page_css_class' which allows plugins to modify the class values included for each page retrieved via wp_list_pages().

[wnorris]

attaching alternate solution, which is more consistent with how this is done in other parts of WordPress... instead of changing $css_class to be an array, leave it as a string, then explode it just for the purposes of passing into the filter. Take yer pick.

[westi]

(In [10814]) Allow a plugin to filter the classes applied in wp_list_pages. Fixes #9304 props willnorris.