Use `wp_get_nav_menus()` instead of `get_terms('nav_menu')` in `WP_Nav_Menu_Widget` to preserve use of `wp_get_nav_menus' filter.

Props Frank Klein.
Fixes #25263.



git-svn-id: https://develop.svn.wordpress.org/trunk@25332 602fd350-edb4-49c9-b593-d223f7449a82