Make the `WP_List_Table::months_dropdown()` query results filterable.

Introduces the `months_dropdown_results` filter.

Fixes #17022.

Built from https://develop.svn.wordpress.org/trunk@25556


git-svn-id: http://core.svn.wordpress.org/trunk@25476 1a063a9b-81f0-0310-95a4-ce76da25c4cd