Introduce previous_image_link and next_image_link filters. Allows customizing...

Introduce previous_image_link and next_image_link filters. Allows customizing the gallery display order.

Props nacin
fixes #22829 for trunk


git-svn-id: https://core.svn.wordpress.org/trunk@23139 1a063a9b-81f0-0310-95a4-ce76da25c4cd