Replace deprecated $wpdb::escape() with esc_sql()
Fix WC_Product_Cat_Dropdown_Walker strict notice

Specifically, fix: "Declaration of WC_Product_Cat_Dropdown_Walker::start_el() should
be compatible with Walker::start_el()".
Merge pull request #3613 from thenbrent/release-2-0-14

Small WP 3.6 patches @coenjacobs can you move to 2.1 as well